If you are definitely getting a bigger tank in three months then I think you should be okay as long as you carry out regular tank maintenance. Check the filtration every couple of weeks as well, you might find this gets dirty very quickly. However, if you do need to rinse out the sponges, make sure you do it in existing tank water and not under tap water.
Just remember the Oscars grow very quickly, in three months time they could be between five and 6 inches, put together, too big for your tank.
Hope that helps