Seeing as you already have the fish in a tank with a filter (and a very small tank at that) i would move it now

Buy a large net. Take some water out of the old tank and put it in a bucket. Put the fish in the bucket

2/3 fill up the new tank with fresh water the same temperature as the old one(use dechlorinator). Get the new tank running with new filter heater etc. Now move the filter from the old tank and put it in the new tank. Then move any gravel from the old tank to the new one, followed by all the old water from the old tank. Fill the new tank up with new water and add the oscar

Have both filters running for 2 or 3 months before removing the old filter (altho you can leave it on the new tank it wont do any harm)

Get yourself a test kit of Ph,ammonia,nitrite and nitrate and test weekly

Return all fish except the oscar to a shop or you will have problems