Oscars are not cave hiders. They tend to occupy open water and fight over feeding grounds and mates. If an oscar is in a cave it means hes stressed out and hiding.
Imo, if you want a jardini tank, jettison the oscars and silver dollars. Ive never kept one, but common wisdom is a jar needs lots of space and its hit or miss as far as them accepting tankmates. I think you have a golden opportunity to grow a nice, fat jar fast in a tank like that by his lonesome. Adding the oscars will only stack up your bioload, strain your maintenance, increase competition for food, and increase stress on your fish once they become mature and territorial. If you keep that water pristine and load him up with good food the jar will be much better off a year from now I'll wager.
If anything i hear jars are pretty tolerant of bottom dwellers so maybe a shoal of pim cats would work.
