The OPC seeks to establish relationships with other faithful churches. It does this through its Committee on Ecumenicity and Interchurch Relations (CEIR). The mandate of the CEIR is to make recommendations to the General Assembly concerning the relationship of the OPC to other churches and to ecumenical bodies both in the United States and abroad. This includes the establishment and maintaining of the official relationships of Churches in Ecclesiastical Fellowship and Churches in Corresponding Relationships.
The document "Biblical Principles of the Unity of the Church" (see link below) sets forth the position of the CEIR and of the OPC regarding the unity of the church.
The OPC is a member of the North American Presbyterian and Reformed Council (NAPARC) and of the International Conference of Reformed Churches (ICRC).