It stands for Ohmygod Poo Inside. Not really very exciting, is it? haha >.>
actually this is the correct information. And sadly not very exciting lol.
What does O.P.I. stand for?
"Other People's Ideas . . . I'm just kidding," said Schaeffer. It is "Odontorium Products, Inc." In the early 1980s, my father took over a dental supply business from my grandfather's cousin. He formulated an acrylic nail product from that background and started out supplying to nail salons on Ventura Boulevard (Los Angeles).