This is one area that's unclear (at least to me). Once you've hit the Basic Healthcare Sum, can you make a voluntary top-up (without triggering a refund without interest) to your Medisave Account after the Basic Healthcare Sum (BHS) increases on January 1 (or after a withdrawal), but before compulsory MA contributions arrive -- as long as you have some room below the CPF Annual Limit, and as long as there's no other contribution in process ahead of your voluntary MA contribution? I don't know, but it seems like that's OK according to the written information CPF provides.
It seems odd to me that MA interest wouldn't be credited to MA first, to account for the BHS rise, then overflow into OA. Is that what you're seeing happen, Suleyman -- that the first dollars of interest go into MA as the BHS rises, then any excess interest into OA? Then again, interest is supposedly booked on December 31 (and posted eventually), then the BHS rises on January 1. So maybe that's how it's supposed to work, in that order, but it's weird.
By the way, for those who have not hit the Full Retirement Sum, and who are below age 55, OA funds (including interest) can be converted to SA if desired.
On edit: OK, this is potentially interesting. If the recipient (which could be yourself) is age 55 or older, and if the recipient has not hit the then current Basic Healthcare Sum (BHS) limit, you can transfer OA or SA funds to Medisave using CPF Form MHC-TRF or online equivalent, as long as you've hit the Full Retirement Sum (or Basic Retirement Sum with property pledge). This transfer does not benefit from tax relief, but it is NOT subject to the CPF Annual Limit. OA funds are obviously better for this purpose (lower interest), and you should make this transfer if you're below the CPF Annual Limit and can make a voluntary contribution with cash for tax relief. (Even if that cash comes from OA or SA, which can be withdrawn from age 55 in this case.) But it is a way to bust the CPF Annual Limit if you're 55 or older, it appears.