OSP, the full name is Organic Solderability Preservative, it's also called preflux. OSP is a kind of surface finish on bare printed circuit board that preserve the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process. It's a kind of surface technology in PCB fabrication that meet RoHS requirements, and it could be alternative solution for the surface finish of HAL(Lead free).
OSP uses a water-based organic compound that selectively bonds to copper and provides an organometallic layer that protects the copper prior to soldering. It's also extremely green environmentally in comparison with the other common lead-free finish, which suffer from either being more toxic or substantially higher energy consumption.
The disadvantages of using OSP surface treatment include: Handling sensitive, short shelf life, Not good for plated through holes(PTH), can cause ICT Issues, exposed Cu on final PCB assembly, no way to measure thickness.