Originally posted by Contra_Mundum
The RPCNA is a church with Covenanter heritage. They are a Psalm-singing church (exclusively); they believe the office of deacon is open to women; they used to be against participation in civil voting due to objections to the atheistic form of US Constitution; they used to be a "dry" church (and their ministers still may be required to commit themselves to abstinence); I believe they still fairly strictly hold to the original Westminster Confession (as interpreted and modifed by their "Testimony" documents.
Other "RPC" churches with different letters after (-GA, -US, other) are different denominations with a smaller footprint on the American scene. Another church with direct Covenanter heritage is the ARP, located mainly in the southern states.
The OPC is a church with a more "mainline" heritage. It was formed by faithful ministers, elders, and members forced out of the chief Presbyterian denomination in the northern states (the PCUSA) in 1936. It has been characterized over its short lifetime by a strong emphasis on rigorous ministerial examination, a representative (federal) General Assembly, a high view of office, and usually a dedication to presuppositional apologetics. It is not an exclusive Psalmody church.
A similar church but with southern states heritage (and its own variations) is the PCA.