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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 in the Ohio Super Lawyers list (2012-2024). She also has been named to the following Super Lawyers lists: Top 100 Attorneys in Ohio, Top 50 Women Attorneys in Ohio, Top 50 Attorneys in Cleveland, and Top 25 Women Attorneys in Cleveland.

Colleen joined Calfee in 1998, and she is a member of the firm’s Diversity, Equity & 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 (2021-2024) and Litigation - Trusts and Estates (2024)
  • Ohio Super Lawyers, Business Litigation (2012-2024)
    • Top 100 Attorneys in Ohio (2024)
    • 50 Women Attorneys in Ohio List (2013-2024)
    • Top 50 Attorneys in Cleveland (2024)
    • Top 25 Women Attorneys in Cleveland List (2013-2018; 2020-2024)
  • Inductee, 2022 Class of the Cleveland State University College of Law Hall of Fame
  • Woman of Professional Excellence Award, YWCA of Greater Cleveland, 2010


J.D., magna cum laude, Cleveland State University College of Law, 1996

B.A., John Carroll University, 1992



Representative Business, Corporate, and Commercial Litigation Matters

  • Serving as lead counsel to an American publicly traded, Fortune 1000, multinational company in a contract and trade secret dispute arising from a previous commercial relationship between the client and a manufacturer based in the Northwest U.S., involving one of the client’s products. The client filed a complaint against the defendant in United States District Court asserting claims for declaratory judgment and breach of implied duty of good faith and fair dealing. A few weeks later, the defendant filed its answer including counterclaims for breach of contract and misappropriation of trade secrets under state law. After discovery commenced, it became clear that the defendant had fraudulently induced and/or breached the contract in a myriad of respects resulting in the client amending its Complaint on two separate occasions to reflect the defendant’s wrongdoing.
  • Serving as lead counsel to a national medication prescription services provider in a proposed class action against them for alleged violations of the Telephone Consumer Protection Act (TCPA). The plaintiff alleges wrongful telephone solicitation of the plaintiff and persons similarly situated in violation of the TCPA and in contravention of the National Do Not Call registry. The client vehemently denies the allegations as it has an extensive TCPA compliance program, and the client asserts that the specific plaintiff provided express consent to the phone solicitation, and therefore both the plaintiff’s claims and the attempted class action certification are meritless.
  • Served as lead counsel in this “bet-the-company” case involving corporate and governance disputes between the Co-Trustees of a family trust that owns a manufacturing and distribution company. Calfee represents the Trustee and shareholder of the client company, who asserted claims for breach of fiduciary duty, declaratory judgment, and allegations of self-dealing against the Co-Trustee after more than a decade of unchecked malfeasance. The claims Calfee brought on behalf of its client sought court intervention to require the Co-Trustee to employ governance and compliance mechanisms to the Trust’s most significant asset, the family company. The defendant Co-Trustee asserted mirror counterclaims yet denied any obligation to employ standard corporate governance procedures to ensure the continued viability of the family owned company. Calfee’s role was to strategize and resolve the impasse as to corporate governance that would set the foundation for the future success of the company and the growth of the Trust assets.

Representative Labor and Employment Litigation Matters

  • Served as counsel for Defendants, a multi-billion-dollar, international construction company in a matter in which the Plaintiff company brought several claims, including breach of duty of loyalty, breach of contract, conspiracy, misappropriation of trade secrets, and interference with prospective business relationships, against (1) two members of a group of its former employees who quit and joined its competitor, Calfee’s client, (2) Calfee’s client company, and (3) a member of the client company’s management team. Upon being served with the Complaint, Calfee mobilized its client’s response to preserve electronic data, seizing all relevant company-issued electronic devices in the possession of Plaintiff’s former employee. Calfee’s clients moved to dismiss the Amended Complaint and the court dismissed five of Plaintiff’s Claims against the client company and dismissed the client management team member from the lawsuit entirely. With just the destruction of evidence and misappropriation of trade secrets claims still pending, the parties engaged in discovery and began taking depositions. Subsequently, the parties signed a settlement agreement in which Calfee’s client paid nothing, there was no confidentiality provision (meaning Calfee’s client was free to discuss the case with its customers), and the lawsuit was dismissed.

Professional & Community

Professional & Community

  • Cleveland State University College of Law Board of Visitors, Member
  • Irish American Law Society of Cleveland, Founding Member, Former Vice President
  • In Counsel With Women, Member 
  • Magnificat High School, Board Member, Chair of Governance Committee 
    • Former Chair of the Board of Directors (May 2017-May 2022)
  • Cleveland State University College of 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® “Best Law Firms” report

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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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