Correction. The PC(USA) was never called the Presbyterian Church of [or in] America. In 1936 the northern Presbyterian Church was called the Presbyterian Church in the United States of America. When the 1936 break away group took the name Presbyterian Church of America, the larger body just didnt like the names being so close, so they sued.
In 1958 the larger northern body adopted the name United Presbyterian Church in the United States of America (UPCUSA).
The larger southern Presbyterian Church was called the Presbyterian Church in the United States (PCUS). In 1973 the current PCA broke away from that group.
The main body north and south churches joined in 1983 to become the (current) Presbyterian Church (U.S.A).
You can track the history visually here. Warning: Itll make your head hurt.
ok, so the OPC broke from the PCUSA, then the PCUSA became the UPCUSA and the PCUS. The the PCUS broke into the PCA and PCUS. Then the PCUS and UPCUSA joined to become the PCUSA. Meanwhile the OPC split into the BPC and EPC. Ouch!