If its been set up since Jan, how did you set it up? Did you just put water in, filter on, heater etc and then put the fish in? Have you been doing water changes regularly, how often did you do them after adding the fish.
Oscars are extremely messy fish and therefore create far more waste (ammonia) than normal community fish. It is unfortunately easy to assume that they have/are suffering from ammonia/nitrIte poisoing, due to lack of cycled tank and living in the extreme ammonia and nitrIte levels the tank has/is going thru during the cycle.
Without a test kit you immediatley need to do a 50% water change, ensuring the tank water temperature is the same as the water already in the tank, remembering to dechlorinate it too. Then go get a test kit (liquid test kit, strips are useless) tomorrow, test the water, if there are any signs of ammonia or nitrIte, you need to more water changes to get the levels to 0, unfortunately if you dont do this quickly you will lose them or cause them permanent damage and the likelyhood is, even if you do, they will still die/suffer.
Be kind to the fish, return them to the LFS, explain you are unable to care for them, frankly they should be ashamed to have sold them to you in the 1st place with that size tank.