Is it OK to build a deck with OSB?
How long do you wish this to last? A week? A year? 3 years? 10 years?

OSB will tend to absorb water. It will come apart when subjected to rain, sun, snow, (repeat). This will take less time to happen than you may like. It is simply not designed to resist the elements for any serious period of time.

But if all you want is something that will work for a few weeks or a couple of months one summer, it will suffice. Just don't expect it to be serviceable as a deck surface in a year or so. I suppose that a good coat of a high quality paint will slow things down, but a deck surface is flat. Snow and rain will collect on it and stay there. This will wet the wood, and keep it wet, a bad thing.

As for those 2x4s, are they treated? Again, if these will be in ground contact (something must touch the ground) then they won't last too many years either. Bugs like untreated wood. Water will get into it, so expect it to rot at some point.