I'm trying to get a sense of scale from looking at the various pics and comparing the Poly's size to the rocks and decor; it seems to me that it could easily get into one of those decor pieces and then manage to get wedged or jammed in place. I've had it happen a number of times with Bristlenose Plecos, and have broken a number of pieces of decor...usually natural driftwood...to get them out. Five days is more than enough time to give them a chance to extricate themselves. All my DIY sponge filters nowadays need to be built with this potential problem in mind as well.
The good news is that your tank is fairly open and easy to search; if he's in there you should be able to find him. Even if he's buried in the gravel...and I'm not a poly guy so don't know if they do that...it'll just take a couple minutes to feel around everywhere and turn him up.
But if you don't find him, either on the floor or in the tank or filter...and if the Oscar hasn't shown signs of having eaten him (trust me, it would show!)...then I think it might be time to break open those decor pieces. They can be fixed easily with epoxy.
Fair warning: running your fingers through all the gravel in the tank, and then removing those decor pieces, may (!) impact that lovely water clarity....but that's what filters are for.