Dennis M. O'Leary

Partner* Admitted to the Ontario Bar: 1984

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*Practising as a professional corporation
  • Bio
  • Awards & Recognition
  • Professional Involvement
  • Selected Speaking Engagements
  • Selected Publications
  • Insights
  • Education

As an experienced litigator and commercial lawyer, Dennis’ clients trust his judgment on pressing energy, environmental and transportation law matters. He draws upon four decades of advocacy experience to obtain the best outcome for his clients’ interests and uses that same experience as a transactions lawyer to draft documents in such a way as to avoid costly and unnecessary steps further down the line.

Dennis is a member of the firm's Litigation, Transportation and Mining Groups as well as Co-Chair of the firm's Energy Group and Environmental Group. He practises in the areas of energy and environmental, transportation, regulation, insurance and commercial litigation. Dennis has appeared before all levels of the courts and numerous administrative tribunals in respect of a broad range of regulatory and litigation matters, many of which involved significant energy, environmental and transportation law matters.

On the energy side, Dennis provides advice and acts as counsel on a wide range of issues, including the Ontario Energy Board in respect of divisional court appeals, as co-counsel to board staff at several rate and facilities hearings and assisting board staff with the development of the draft codes for use in Ontario's deregulated electricity sector. He acted as the facilitator at the board mandated settlement conference held as part of an application by one of Ontario's major utilities for approval of its rates. Dennis has appeared before the National Energy Board in respect of toll and facilities hearings. He provides legal advice to international utilities on the regulation of electricity and natural gas at both the federal and provincial levels.  

His environmental experience includes acting for clients with respect to applications for landfills and other waste facilities, responding to spills and other discharges of contaminants, commencing or defending civil lawsuits and defending environmental prosecutions. Dennis has acted as counsel to significant parties in several large environmental assessments, including the Timber Management Class Environmental Assessments and the Ontario Hydro Supply/Demand hearing. 

Dennis has a strong understanding of the specialized legal and regulatory environment that comprises Indigenous law. He has assisted clients in negotiations with Indigenous communities in respect of projects and applications, including the lengthy Timber Management Class Environmental Assessment Hearing.

From a transportation law perspective, Dennis advises clients in respect to freight and warehouse claims, the defence of personal injury claims and how to meet the requirements of the Transportation of Dangerous Goods Act and Regulations. He has represented transportation clients in respect of regulatory offences and in regard to general commercial litigation. Dennis has also assisted transportation clients involved in commercial transactions from an environmental, regulatory and litigation perspective.

Dennis' clients include national and international companies, organizations, insurers, financial institutions and individuals. Dennis has also successfully acted for many clients making use of alternative dispute resolution formats.

Awards & Recognition

  • Recognized in The Canadian Legal Lexpert Directory as a leading lawyer in the area of Energy (Electricity)
  • Recognized in The Best Lawyers in Canada in Energy Regulatory Law and Environmental Law
  • Recommended in The Legal 500 Canada for his expertise in the practice areas of Energy: Power and Environment

Professional Involvement

Professional Activities

  • Counsel to Enbridge Gas Distribution Inc. for Demand Side Measures (DSM) matters, including appearing at the DSM Generic Proceeding and annual account clearances.
  • City of Toronto Contaminated Lands Task Force, 1995-1997
  • Member of the legislative sub-committee of the CBA(O) Environmental Section examining and contributing to several submissions made to the Ministry of the Environment and Energy in respect of amendments to environmental legislation in Ontario, October 1994-1997
  • Member of the Lynn McLeod Strategic Environmental Policy Working Group, December 1994 to the spring of 1995 which developed the environmental platform for the 1995 general provincial election
  • Chair of the CBA(O) Environmental Law Executives' Subcommittee and contributor to the Submission to the Government of Ontario in respect of the proposed amendments to the Statutory Powers Procedure Act, November 1994
  • Proposed Amendments to the Trees Act - A Submission to the Ministry of Natural Resources, July 1992


  • Association of Power Producers of Ontario
  • The Advocates' Society
  • Canadian/Ontario Bar Association (Environmental Law Section, Past Executive 1990-2001)
  • Canadian Transport Lawyers Association (CTLA)
  • Transportation Lawyers Association

Selected Speaking Engagements

Co-presenter with Michael Lyle, General Counsel, Ontario Power Authority, entitled "Buying Into the Benefits of Sustainability and Conservation: New Developments and the Energy Act," Canadian Institute's 8th Annual Environmental Law & Regulation in Ontario Conference, Sutton Place Hotel, Toronto, November 2005.

Speaker, "Environmental Review of Energy Projects," Insight Information Co. - 2nd Annual Environmental Law and Compliance in Ontario, St. Andrew's Club and Conference Centre, Toronto, February 2005.

Speaker, "An Update on Federal Environmental Activity," University of Toronto/EPIC: Understanding Environmental Regulations - Due Diligence Conference, Sheraton Four Points Hotel, Mississauga, October 1998.

Speaker, "Drive Clean Program: An Update," published in Environews, The CBA(O) Environmental Subsection Newsletter, October 1998.

Speaker, "An Update on Federal Environmental Activity," Understanding Environmental Regulations - Due Diligence Conference, University of Toronto/Epic conference, Valhalla Inn, Toronto, October 1997.

Chair, four panel sessions; each panel considered issues within the context of one of four subject areas which are relevant from the perspective of the characterization and remediation of contaminated sites, including regulatory, business, technical and public interest. 90th Annual Meeting and Exhibition of the Air & Waste Management Association, Toronto, June 1997.

Organizer and chair, of the CBA(O) Section Meeting, "Turning Brownfields into Green: Remediating Contaminated Properties," with panellists Christopher Morgan, Ph.D., of the City of Toronto, and Beth Benson, of the Waterfront Regeneration Trust. Toronto, May 1997.

Speaker, "A Review of Federal Environmental Law," Undertaking Environmental Regulations - Due Diligence Conference, University of Toronto/EPIC conference, Valhalla Inn, Toronto, October 1996.

Co-presentation with Chris Morgan, Ph.D., City of Toronto Planner on "Site Decommissioning and Remediation Strategies." Delivered a presentation entitled "An Update on Federal Environmental Activity," September 1996.

Chair and speaker, "Canadian Environmental Health & Safety Management Conference'" sponsored by Executive Enterprises, Marriott Hotel, Toronto, September 1996.

Speaker, "Environmental Liability of Directors & Officers," Canadian Institute conference, Crowne Plaza Hotel, Toronto, September 1996.

Speaker, "Recent Federal Environmental Activity-March 1996," Executive Enterprises Environmental seminar, Sutton Place Hotel, Toronto, April 1996.

Speaker and seminar leader at workshops held for senior lending officers of a major Canadian Chartered Bank in respect of environmental issues affecting lending institutions, March and June 1995, and February 1996.

Speaker, "Which way to Swan Hills? A Road Map for Hazardous Waste," 1996 CBA(O) Institute for Continuing Legal Education, Environmental Law Program, January 1996.

Organizer and Master of Ceremonies of the CBA(O) Section meeting, "An Evening with the Deputy Prime Minister and Minister of the Environment, The Honourable Sheila Copps," Toronto, November 1995.

Speaker, "Federal Environmental Regulation-An Update," University of Toronto/EPIC Understanding Environmental Regulations seminar, Holiday Inn Airport South, Toronto, October 1995.

Speaker, "Recent Federal Environmental Activity," Executive Enterprises Environmental seminar, Sutton Place Hotel, Toronto, September 1995.

Speaker, "Disposal of Hazardous Waste at Swan Hills, Alberta:-Transportation of Dangerous Goods and other Regulatory Issues," CBA(O) Section Meeting, May 1995.

Organizer and co-moderator of the CBA(O) Section Meeting "The Ontario Political 'Environment'," a debate about topical environmental issues featuring the Minister of the Environment & Energy and the environment critics for the other two provincial political parties, Toronto, March 1995.

Speaker, "Legal Liabilities for Landfill Leachate and Gas Management," University of Toronto/EPIC seminar, April 1994.

Panellist, "The Environmental Bill of Rights," joint MOEE-CIELAP Public Forum hosted by Steve Paikin of TV Ontario, Innis Town Hall, Innis College, March 1994.

Speaker, "The Environmental Bill of Rights in the Year 2000," Canadian Institute for Environmental Law & Policy (CIELAP), Hart House, University of Toronto, March 1994.

Organizer and chair of the CBA(O) Section meeting entitled "Property With No Value May Be Worth Something - Contamination and Municipal Tax Assessments," Toronto, March 1994.

Co-presenter with H. Kohn, "The Environmental Bill of Rights-A Municipal Primer," Municipal Health & Safety Representatives Association Workshop, November 1993.

Speaker, "How to Use the Law for Conservation," 65th Annual Convention of the Ontario Federation of Anglers and Hunters delivered jointly with Mr. Ed Hanna of J.E. Hanna & Associates, February 1993.

Speaker, "Ontario Municipalities: Legal Consequences of Waste Illegally Dumped onto Municipal Property," to several municipalities, June 1990.

Selected Publications

Author, "Ontario Energy Board Announces Oral Hearing to Review Cost of Capital," Aird & Berlis LLP, Energy Insider, March 13, 2024.

Co-Author, with Robert Doumani and Tom Halinski, "Hydro De-Regulation and the Tenant Protection Act," January 2001 (updated October 2001).

"Update on the 'Clear Language' TDG Regulations," Environews, The CBA(O) Environmental Subsection Newsletter, October 1999.

"MOEE Announces Regulatory Standards for Landfills" and "More Deadlines for Underground Fuel Storage Tanks," Environews, the CBA(O) Environmental Subsection Newsletter, October 1996.

"Gasoline Handling Code and Underground Used Oil Tanks," Environews, the CBA(O) Environmental Subsection Newsletter, November 1995.

"Must I Pull That Used Underground Storage Tank?," Ontario Automobile Dealer Association Newsletter, November 1995.

"But What Exactly is a Class Action," HeCS Newsletter, December 1994.

"Contaminated Property-How to Lower the Risk - An Update," Business Reorganization Newsletter, June 1994.

"Construction & The 3R Regulations," The Construction Law Letter, Build/Law Publications, Vol. 10, No. 2, circulated to members of the Ontario General Contractors Association, November/December 1993.

"Proposed 3R Regulations under the Environmental Protection Act," for the CBA(O) Environmental and Municipal Law Executives' Waste Reduction Subcommittee and submission to the Ministry of Environment and Energy, June 1993.

"New Federal Emission Reporting Requirements The National Pollutant Inventory," The Environmental Law Report, April 1993.

"Environmental Audits and Solicitor-Client Privilege," The Environmental Law Report, December 1992.


  • LLB, Queen's University, 1982
  • BA, University of Toronto, 1979