Charter Oak Federal Credit Union has been open since 1939. It's the 3rd largest credit union in Connecticut with assets totaling $1.16 Billion and providing banking services to more than 76,000 members. Membership: The credit union is a community-based financial institution. People living, working, or worshipping in the credit union's geographic area can become a member and access banking services. Membership is also open to immediate family of current members.