aggreed with Mr whiskers, shape and size of the tankmates can be important.
O's are very greedy and will try to eat any fish that fits.
I saw that happen.
My O has been living with 6 Silver Dollars and they're super tankmates.
My O chases them around and keeps them schooled up .
They clean up all mess my O makes, that's food and poo...
I introduced a small barb some months ago, and it's been living as a tankmate with my O now.
So smaller tankmates that fit in the mouth of your O can disappear, or live happily together.