You are not alone.
Oscars settling down to the bottom of the tank is something they often do, for any number of reasons. Just get used to it. It can be the result of stress or insecurity. There are about one hundred ways to make him lie down, among them:
---Eating Too Much. When he's too full, he's on the bottom.
---Playing Dead. One of the ways Oscars hunt in the wild is
to lie in the sand, and then they attack unsuspecting meals.
---New Surroundings. In a new aquarium or with a lot of new
decorations, bloop! To the bottom.
---Incorrect Aquarium Temperature. Gravel dweller.
---New fish that are larger than him. Submarine City.
---Lost a friend. One of his tankmates died or disappeared.
Like a rock, to the bottom.
---Hurt Feelings. When he follows you the entire length of that
48" aquarium and you don't stop to say 'hi'--Lead weight.
---Sleeping. An average Oscar sleeps approximately 8 hours a
night -- you guessed it, on the floor.
---Poor Water Conditions. Glug, glug, glug!
As you can see, an Oscar is a very emotional animal and is unafraid
to tell you when there is a problem. If I see an Oscar down on the floor
of the aquarium, the first thing I do is test the water quality (ammonia
and nitrite) and the temperature. If these tests show no problem, then
something may be making him insecure or he's eaten too much. Has there
been a big change in his life recently? Or are those 80 Rosy Reds you got
him ALREADY GONE?!?
Anyway, if your Oscar becomes a bottom dweller the first thing to understand
is that this behavior is perfectly normal.