Mr Dennis M. O'Leary > Aird & Berlis LLP > Toronto, Canada > Lawyer Profile

Aird & Berlis LLP

Work Department

Litigation, Energy, Environmental, Transportation, Insurance


Dennis is a member of the firm’s Litigation, Environmental, Transportation and Mining Groups as well as Co-Chair of the firm’s Energy Group.


Dennis 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. 

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.


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


LL.B., Queen’s University, 1982
B.A., University of Toronto, 1979

Lawyer Rankings

Canada > Environment

Aird & Berlis LLP is has experience advising both public and private sector clients on environmental law and other related matters, including Aboriginal law, brownfield redevelopment, contaminated sites, climate change and environmental compliance. Toronto-based litigators Scott Stoll and Dennis O’Leary are key figures in the team.