Dude
You are not getting any sympathy from us, and your excuses are not appealing to us. You should have found out how big they get before getting even just one. And, most importantly,
YOU ARE NOT EVEN TALKING ABOUT RESOLVING THIS
You are talking about "seeing how it goes" -- That is very, very immature, ignorant, on your part. Maybe it is true that you just found out yesterday, but what you are saying, is that you are keeping the tank as-is.
Do the right thing, and IMMEDIATELY remove ALL the oscars and koi. Maybe if they are tiny keep one, but ONLY IF, and ONLY IF, you have a BIG tank ready within a few months.
Otherwise, GET RID OF THEM ALL NOW. Why make the fish suffer by having the problems "in the future" that you speak of.
No offense, truly, but I have heard quite a few Indians express a clear disregard for animals and their needs. If you are any different, then you will follow our advice IMMEDIATELY.
Sorry for venting, but your posts speak for themselves. That, in my opinion, is not welcome. We, as fishkeepers, accept that singular responsibility for their well-being and health, which you clearly aren't doing.