Kathleen O'Neill

Full contact info

I focus on antitrust and competition law, and I advise our clients on all aspects of government merger review, antitrust investigations, litigation and compliance.

About Kathleen

Kathy advises clients on all aspects of antitrust and competition law, after serving 15 years with the US Department of Justice Antitrust Division, most recently as senior director of investigations and litigation, the top career civil position in the Antitrust Division. In that role, Kathy oversaw all merger and civil conduct investigations and litigations across industries. She previously served as chief of the Transportation, Energy and Agriculture Section of the Antitrust Division, and earlier as a trial attorney in the Telecommunications and Media Section, among other positions.

During her tenure in the Antitrust Division, Kathy played a key role in a number of high-profile merger reviews, anti-competitive conduct investigations and litigation across a range of industries, including the airline, communications, technology, entertainment, energy and agriculture sectors, among others. She oversaw high-stakes investigations and litigation involving tech firms, and she helped pioneer the DOJ’s approach to challenging acquisitions by dominant firms, overseeing litigated challenges involving nascent technology.

Kathy also has experience with labor issues and played an integral role in advancing the DOJ’s merger challenge in the publishing industry involving monopsony allegations in labor markets. Kathy negotiated settlements resolving the DOJ’s competitive concerns, including a large and complex merger settlement in the genetically modified seed sector.

In addition, Kathy has extensive experience enforcing Hart-Scott-Rodino (HSR) Act violations, including serving as the litigation lead on a failure to file matter that resulted in the largest settlement on record for an HSR violation involving the “solely for the purpose of investment” HSR exemption.

While at the DOJ, Kathy worked closely with senior attorneys at the Federal Trade Commission, competition enforcers in other countries, and state attorneys general. She also represented the Antitrust Division in congressional briefings and worked on policy initiatives, including President Joe Biden’s July 2021 Executive Order on Promoting Competition in the American Economy. She also worked on revising guidance documents, such as the government’s merger guidelines and the Antitrust Division’s Merger Remedies Manual.

Kathy is an eight-time recipient of the Antitrust Division’s Assistant Attorney General Award for her role in enforcement actions in the advertising, agriculture, airline, energy and wireless sectors. She also was recognized by Women@Competition in 2019 as one of its 40 in their 40s Notable Women Competition Professionals in Europe, the Americas and Africa. 

Prior to joining the DOJ, Kathy served as an assistant attorney general for the New York State Attorney General’s Office and as an attorney advisor at the Federal Communications Commission and in private practice.

Speaking engagements

  • Panelist on “Initial HSR Waiting Period,” “Agency Front Office Advocacy, Litigation, and Remedy” and “Voting, Final Meeting,” American Bar Association Mergers Workshop, May 2022
  • Panelist on “Mergers: Everything Old is New Again: The Return of the Vertical Merger,” Global Competition Review Live: Law Leaders Global, January 2022
  • Panelist on “Competition and Data Issues in Digital Advertising Markets,” Korea Fair Trade Commission’s 11th Annual Seoul International Competition Forum, November 2021
  • Panelist on “Federal and State Antitrust Enforcement,” Practicing Law Institute, Antitrust Counseling & Compliance 2021, October 2021
  • Panelist on “Antitrust Under the New Administration,” Women’s Antitrust Forum, October 2021
  • Panelist on “Merger Enforcement,” Georgetown Law’s 15th Annual Global Antitrust Enforcement Symposium, September 2021
  • Panelist on “Mergers,” Antitrust & Competition in Key Industry Segments virtual conference hosted by Sandpiper Partners, July 2021
  • Panelist on “Agency Update With the US Department of Justice,” American Bar Association Antitrust Spring Meeting, March 2021


Tulane Law School
JD, 1998, cum laude

University of Pennsylvania
BA, 1994, English, with distinction

Court admissions

US District Court for the District of Columbia

US District Court for the Northern District of California

US District Court for the Southern District of New York

Rankings and accolades

Chambers USA: Antitrust – District of Columbia (2024)

Women@Competition: 40 in their 40s Notable Women Competition Professionals in Europe, the Americas and Africa (2019)

Global Competition Review: Women in Antitrust award (2016)

DOJ Antitrust Division: Assistant Attorney General Award for outstanding contributions to enforcement matters (eight-time winner)