I have a 400+ gallon indoor pond. I already have ten 6-10 inch long koi carps and three 8 inch long catfish (Iridescent Shark). And I'm deeply fond of oscars. Would it be okay if i add two oscars to the same pond.

Well, I would say you are probably at your limit already. Koi carp have the potential to grow to 30lb +. They will not reach this size is an indoor aquarium but they will run much much bigger than an Oscar. They are also cold water fish whereas Oscars are tropical.

So it's probably not a good idea to mix the two fish

i am in a tropical country and have seen mix up
of oscar and koi doing well.. however, i am not sure
koi and ID shark, though i am presently have two huge
id sharks with 3 oscars without koi.

but we can Ice skate in winter!

I wouldnt add anything else to the tank the koi are messy enough you will have trouble keeping the water paramiters at correct levels,however they will live together the oscars will be ok as low as 18c but will grow slowly,but the carp will alsaw be ok at that temp but they grow quicker at high temps...Plenty of air stone will be needed and filteration will have to be huge!!

thanx guys. i'm planing to give away some of my koi carps because they are too messy for an indoor pond. I'll be keeping only 2 of them. And hoping to add two more white ID sharks.