my problem is that i have two oscars in a 90 gallon tank and they are always fighting scince last week. i have had them for over a year now and i dont know wat to do:S

sounds like you have 2 males coming to maturity and fighting for dominance ...you could divide the tank with plexiglas if its serious but if its not to bad its better to let them sort out their differences....

i notced thatt it is only at night i hear big sollashing noises

Like biggamehunter said, you probably have 2 males, all you can do is either let them get on with it and hope they dont cause to much damage to each other because then you'll have to split them.
or split the tank into 2 with a divider.
how big are your oscars? and do you have any other fish in your tank?

night splashing should also be observed very keenly.
there are times parasite cause their uneasiness. if
they are fighting for dominance, they will do it every
time, not in night time only. please check water parameters;

if they only make noise at night it might not be a dominace thing. as paul pointed out if it was dominace they would be at it all the time. do you have any nocturnal fish in your tank with your oscars? can you let us know what other tankmate you have?

night active fishes can create chaos in the tank.

The fish i have are 2 parrot fish 1jack dempsey 1 convict and one pleco

there is 1 jack dempsey 1 convict 1 pleco 2 parrots

the pleco is probably the culprit. they are noturnal and your is more than likely startling your oscar when the lights go out.

take note also that big pleco could take bigger oscar.
i have an incident with one of my client wherein a pleco
was able to anchor that sharp sucking mouth on the side of
an sleeping oscar and eventually kill it.

my oscar at night used his head broke the tank top off and jumped about 3 feet lucely he is still alive