Bonus interest mostly goes into SA (although some can end up in MA, especially earlier in a working career). OA can contribute up to $20,000 toward hitting the $60,000 total combined balances figure to earn maximum bonus interest, but bonus interest computed on OA is credited to SA (or to RA from age 55).
I think we're also forgetting the fairly typical/conventional 13th bonus month, and we're also forgetting possible "spillover" from MA into SA once MA reaches the Basic Healthcare Sum. The allocation rate into MediSave is higher except for age 50 to 55, and MA earns 4% interest, so as long as MA withdrawals aren't too significant there's a realistic prospect of some "spillover."