The business will specialise in digital marketing, advising marketers who want to move their businesses into the online space as well as handling pitches for digital accounts.
Cowie has left his role as the chief executive of the WPP-owned events business FITCHLive to join Thompson in the new venture.
Oystercatchers has signed a strategic alliance with Agency Assessments International, giving access to the intermediary's clients around the world. It will also take over all of the existing clients from Suki Thompson and Associates, including lastminute.com.
Thompson said: "The digital revolution is gathering pace, we can help clients with the whole process of change."
The duo is backed by a team that includes Josie Jacobs, the head of advertising and brands at Tourism Australia, Richard Buitenhek, the vice-president of marketing thought leadership at BP, and Fran Cassidy, the Carlton TV media specialist.