Of course, much depends on whether you are asking someone from the OPC or the PCA. ;-) They are the same in official doctrine. They both adhere to the Westminster Confession of Faith. There are some slight differences in the PCA and OPC Book of Church Order, which states how things should be run.
The OPC was formed in the late 1930s when J. Gresham Machen and some other men took a stand against the increasing liberalism of the main Presbyterian body. They lost a great deal personally in leaving and forming a new denomination. The PCA was formed in the 1970s when the northern and southern branches of the main Presbyterian denomination agreed to join. The church was becoming even more liberal, and many people left to form the PCA.
The OPC tends to be more formal and conservative in its worship services (some in the PCA would say stodgy, I suppose). There are more OP churches in the north and more PCA in the south, although both are expanding into new territory.
Just an aside, Pastor Bruce Hunt was an OP pastor who stood up to the Japanese in Korea and was imprisoned for it. The Koreans were so impressed by his courage that the Presbyterian church in Korea flourished. http://www.wts.edu/resources/sarang/brucehunt.html
Hope that helps.
Thanks! I just looked up - it seems that the OPC also does not have women pastors or deaconesses while the PCA is heading to the latter.