Over at Of Trout and Men, my friend and mentor Dan Knox has a helpful little synopsis answering an inquiry concerning the difference between the different Presbyterian denominations of the Orthodox Presbyterian Church (OPC), the Evangelical Presbyterian Church (EPC) and the Presbyterian Church in the United States of America (PCUSA).
The person posing the question to Dan did not include the Presbyterian Church in America (PCA), but the PCA is quite similar to the OPC, so you can include the PCA with the OPC in that article.
But the OPC and PCA are still distinct. You can read of these distinctions in a helpful article by Peter Wallace. Wallace is uniquely qualified to answer this question given that he is an OPC pastor who has been serving a PCA congregation for over six years. As such, Wallace attends both Presbytery meetings for the OPC and PCA, as well as serves on the Committee on Ecumenicity and Interchurch Relations, where his duties have included serving as a fraternal delegate to the PCA General Assembly, as well as some face-to-face discussions with representatives of the PCA's Interchurch Relations Committee. Wallace provides a helpful, fair and balanced perspective that doesn't get caught up in the extremes.
You want to know the difference, then check out these articles.