Oklahoma:

Prominent theatre companies in Oklahoma include, in the capital city, Oklahoma City Theatre Company, Carpenter Square Theatre, Oklahoma Shakespeare in the Park, and CityRep. CityRep is a professional company affording equity points to those performers and technical theatre professionals. In Tulsa, Oklahoma's oldest resident professional company is American Theatre Company, and Theatre Tulsa is the oldest community theatre company west of the Mississippi. Other companies in Tulsa include Heller Theatre and Tulsa Spotlight Theater. The cities of Norman, Lawton, and Stillwater, among others, also host well-reviewed community theatre companies.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What is the oldest resident professional theater company in Oklahoma?
American Theatre Company