yes the tank is too small- get on ebay and look for a 300L min, and what hes doing is normal, they are very moody fish and hate anything new,,, trust me on that!
more to the point anyway... did you research oscars before buying one? they are big ( can get up to 14inches) messy buggers, they live for up to 10yrs and require massive filtration on a tank,,, go to the oscar club in this section and at the very top post nelly has kindly linked to a tonnes of usefull information... which you should have read before buying him

im not being unfriendly, but fed up of seeing 10 inch oscars looking miserable in small fish shop tank because whoever bought them didnt bother learn the basics about them first,, ( im just about to rehome a huge one that had outgrown his tank)
can you provide the care it needs
can you upgrade the tank ( bearing in mind that at 14 possible inches it will need to turn around freely
provide the filtration
you put him in with guppies

??? what on earth were u thinking? sounds very much like an impulse buy to me
on a lighter note, i have kept oscars for almost a year now and totally smitten, they are beautiful intelligent fish who will have you wrapped around there little fin in no time,, they are not hard to look after . providing your willing to put in the effort
there is tonnes of advice on here on anything could imagine, we have nothing to gain from advising you and can asure you we only havw the fishes welfare at heart, and your sanity (( youll find out what i mean about that in a few months

))
so, does he have a name, and what are you planning on feeding him? would love to see a photo when hes settled in
best of luck
shelagh xxx