OP stands for Original Pronunciation – so Shakespeare in OP would be to speak the lines of Shakespeare’s plays in an accent that would be the same way audiences would have heard the plays in the late 1500’s.  It’s not the same as a contemporary English accent, and it certainly isn’t modern American pronunciation.

This link is to an article about a production of Midsummer Night’s Dream to be performed entirely in OP, a result of the research of linguist David Crystal and director Paul Meier, and it contains a couple of short videos which demonstrate the sound of the accent.