When he gets 5-6" get a wider tank. 1.5 feet or wider. Plecos are fine. Plecos rarel;y rarely rarely exceed 24" in a tank smaller than 180G. Commons and sailfins grow large but they normally top out at 18". Bristlenose plecs top out at 6" and are one of the best plecs to have.
The oscar will not try and eat the plec unless its really small, like under 1".
congrats on the oscar lovely looking bugger i would go for a larger tank......seen a oscar in a aquaruim at a oceanauim in a 4ft tank and he looked unhappy they do need space the biggest you can go the better
thanks for the comments, im aware of the size these great fish exceed to, and will hopefully buy him a 75gal, with maybe another tank mate, what do you think?
I forgot, You could add maybe a shoal of silver dollars? But they look a bit dull. Its up to you, sort of. The ocsar has a say as to what fish go in aswell.