ASM 300 OA, means Amazing Spiderman 300 "Original Art". Original Art in this case would mean the original art of the cover of ASM 300 that was used to print/produce the comic----- just meaning the first Art that the comics were made from...... there is an Original Art thread/forum here on the CGC boards, where people discuss what they've bought and sold of the blueprints of comics whether it be a splash page ----a comic page of OA with no words, -------- OA of a cover,,,,,,or OA with word bubbles.
I think this is what you were asking information on; Anyway hope it helps @grayzr