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Colleen has extensive experience in federal and state courts nationwide helping business clients resolve the following:

  • Commercial contract disputes
  • Shareholder and corporate disputes, including post-closing issues
  • Estate and probate matters
  • Trade secret claims
  • Real property disputes
  • Municipal and public law issues
  • Title VII claims

Colleen has counseled and represented businesses, financial institutions and high-net-worth individuals in a variety of corporate, commercial, fiduciary and employment disputes with a results-oriented, practical approach.  

She has successfully prosecuted and defended a variety of business tort claims in both federal and state courts nationwide.

Colleen has represented municipalities and clients in the energy industry in various types of litigation, including contractual disputes, real estate disputes and employment matters.

Colleen has been selected for inclusion to the Ohio Super Lawyers list from 2012-2021. She has also been named to the Top 50 Women Ohio Super Lawyers list from 2013-2021 and as a Top 25 Women Cleveland Super Lawyers list from 2013-2018 and 2020-2021.

She joined Calfee in 1998, and she is a member of the firm’s Practice Planning and Growth Committee and the Diversity & Inclusion Committee. Colleen began serving as Vice Chair of the firm's Litigation practice group in 2019 and Co-Chair in 2021.

Honors & Recognitions

  • The Best Lawyers in America©, Commercial Litigation (2021-2022)
  • Ohio Super Lawyers (2012-2021)
    • Top 25 Women Lawyers in Cleveland List (2013-2018; 2020-2021)
    • Top 50 Women Lawyers in Ohio List (2013-2021)
    • Top 100 Ohio Super Lawyers List (2013-2015)
  • Woman of Professional Excellence Award, YWCA of Greater Cleveland, 2010


J.D., magna cum laude, Cleveland-Marshall College of Law, 1996

B.A., John Carroll University, 1992

Professional & Community

Professional & Community

  • Cleveland-Marshall College of Law Board of Visitors, Member
  • Irish American Law Society of Cleveland, Founding Member, Former Vice President
  • Magnificat High School, Chair, Board of Directors (May 2017-present)
  • Cleveland Marshall Law Alumni Association, Board Member
  • Leadership Cleveland Class of 2016
  • Cleveland Metropolitan Bar Association, Member
  • Ohio State Bar Association, Member
  • American Bar Association and the ABA’s Litigation Section, Member



  • "Trade Secret Considerations to Avoid Business Risks," Calfee's 2019 Intellectual Property Seminar, Cleveland, OH (June 27, 2019)
  • "2018 Litigation Trends and Regulatory Changes," Calfee's 19th Annual Litigation Seminar and CLE presentation, with Mitch Blair and David Bules, Cleveland, OH (September 20, 2018)
  • "2017 Litigation Trends and Regulatory Changes," Calfee's 18th Annual Litigation Seminar and CLE presentation, Cleveland, OH (September 13, 2017) 
  • “Mitigating Litigation Risk in Trust Administration,” National Trust Real Estate Association (May 3, 2017)
  • "Privacy and Data Security: 10 Steps for Minimizing Cyber Threats," Calfee's 16th Annual Litigation Seminar and CLE presentation, with Jamie Ramsey and K. James Sullivan, Cleveland, OH (September 16, 2015)
  • “Litigation Basics,” Ohio State Bar Association CLE Presentation
  • “U.S. Supreme Court Update with Emphasis on Class Action and Whistleblower/Retaliation Developments, including Dodd-Frank,” Calfee Complex Litigation Seminar, Cleveland, OH (July 2011)
  • “Dealing With Difficult Depositions,” Ohio State Bar Association District 5 Annual Meeting (October 11, 2010)
  • “A.S.O.R.T. Training: Recent Operations and Lessons Learned,” Allied Special Operations Response Team, with Rick Goddard (February 2010)
  • “Law Enforcement and Civil Rights Litigation Training Seminar,” with Maura L. Hughes (February 2010)

Client Feedback

Client Feedback

"I originally engaged Colleen O’Neil of Calfee, Halter & Griswold to provide local representation in an arbitration where the venue of Cleveland had been previously agreed to. Initially, the selection of Colleen O’Neil came from research by my California counsel identifying potential candidates (essentially referrals where my CA counsel started with around 6-7 potentials, culled that down to three, and then homed in on recommending the best candidate; that selection included taking into account both Calfee’s size and reputation). Colleen has proven herself to be an outstanding litigator, but she has ADDED significant value to her original and continuing role as a litigator by bringing in other attorneys at Calfee to handle additional work for me. Calfee is now my corporate counsel." 

– Calfee client, as reported by U.S. News – Best Lawyers® in the 2019 “Best Law Firms” rankings report

News & Events


Licensed In

  • Ohio

Court Admissions

  • U.S. District Court for the Northern District of Ohio
  • U.S. District Court for the Southern District of Ohio
  • U.S. Court of Appeals for the First Circuit
  • U.S. Court of Appeals for the Fifth Circuit
  • U.S. Court of Appeals for the Sixth Circuit
  • U.S. Court of Appeals for the Eighth Circuit
  • U.S. Supreme Court
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