Camden Bucey's take on it, I sum it so:
The PCA was founded in 1973 after many conservatives left the progressive Presbyterian Church in the United States (PCUS), which is often called the “Southern Presbyterian church.” The OPC was founded in 1936 out of the Presbyterian Church (USA), the mainline Presbyterian body of the north.
PCA = More evangelical. Missionaries do their own fundraising.
OPC = More as a pilgrim. It appears that Van Til (or so they claim) considered modern evangelism as Arminianism. I think it's taking it too far, since Van Til also went out to preach on the streets. Missionaries fully supported by the church/denomination.
So this is why I don't sense the OPC really do evangelism and even objected to it.