You need to take the Oscar back, right now, its probably suffering ammonia and nitrIte poisoning, due to the fact that the tank isnt fully cycled, at 2 weeks its likely at a high ammonia level with nitrIte rising steadily. You ask why the plec isnt behaving that way, because its been slowly acclimatising to the bad water quality, the Oscar not being used to it, is suffering. In the long run it will kill or seriously damage the health of both fish.
In either case, a 30g long tank is NOT large enough for an Oscar, it maybe small now, but it wont be in 4/5 month time, an Oscar, feed properly and kept in good conditions, will grow 1"+ a month! It needs to be in a minimum of a 4ft long, 18" wide, 18" deep tank for long term health and growth.
Please, whilst you may like this fish, you clearly didnt research before buying (which is sadly often the case) and for the sake or the Oscar (and plec long term) return him, damage is more likely already caused, the least you can do now is make things easier for it.
