I would say depends on body type. I would say a 4" silver dollar would be safe but a 4" bala shark might be lunch. I would go with red hooks, tinfoil barbs or other larger growing barbs if your tank is big enough.
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.