The Orthodox Presbyterian Church is also a Presbyterian church. In its essence, this means that we are confessional and connectional. . . .
We are connectional. We also express our connectional character by our intentional structure. Congregations in the Orthodox Presbyterian Church are led by their elders, who serve on the local (session), regional (presbytery) and national (general assembly) levels. At every stage, our church government provides accountability and connects each congregation to the worldwide mission of the church. Together, we seek to establish the worship of God and take the gospel to all people everywhere.
The question left unanswered here is how does the OPC understand its relationship to other denominations in light of its presbyterianism? The implication of this collegial and catholic view of the unity of the church is that denominational separation involves a mutual charge of schism and a mutual rejection of each others' de jure legitimacy and authority. Thus, the OPC, in remaining separate from other denominations such as the PCA or the FPCS, is charging these other denominations with schism and rejecting their objective legality. The OPC, therefore, ought to make this position more clear and explicit. If it doesn't want to affirm this, then it needs to unite immediately with other denominations whose legality it accepts in order to honor the catholicity and collegiality of the visible church. If it doesn't want to do either of these things, then it needs to stop pretending to be a Presbyterian church.
Since the OPC has clearly affirmed that it holds to Presbyterianism (and has written some great stuff on this point, such as this document), I will assume until I have clear reason to do otherwise that it understands (at least on paper) its relationship with other denominations in a presbyterian manner, even while calling it to do better at expressing its position clearly and consistently.
For more, see here, here, and here.