I hv some questions.
1. was told that after 55yrs, there will be no more OA, SA.
just simply OA and RA, didn't know that SA still continue.
Are you asking if your understanding is correct, that individuals age 55 and over can have Special Account balances, even big and still growing ones? Yes, they can and often do.
2. If at 55yrs, set aside RA full amount of ERS 279k, means commit to ERS or still able to choose BRS later?
Top up dollars (and interest on those dollars) are distributed via monthly payouts when you decide to start those payouts (which can be at age 65, age 70, or any time in between). If you top up your Retirement Account to the Enhanced Retirement Sum then your monthly payout amount will certainly be substantially more than it would be at the BRS level.
However, it's a bit unclear whether you can top up to the ERS (on your 55th birthday for example), make a property pledge just before payout start, and then receive monthly payouts that are above the BRS level but below the ERS level -- at or about the FRS level, for example. The published rules suggest that particular combination is possible (ERS top up, ~FRS level payout), but if you're concerned you should ask CPF directly.
Note that previous voluntary top ups are also paid out monthly in annuity form. If, for example, your Retirement Account is formed and funded on your 55th birthday to the Full Retirement Sum, and if 30% of that RA is attributable to previous voluntary top ups,
and and if you then top up your RA to the ERS (hopefully within the same calendar month your RA is created so that you can immediately start collecting interest), your future monthly annuity should be somewhere at least in between the FRS and ERS level when you make a property pledge. But again, ask CPF directly for the specific numbers in your specific situation.
3. Assuming can choose BRS later, means will hv 4 accounts OA, SA, RA and BRS? any cash withdrawal will start from RA first?
There's no "BRS account." (Your fourth account is your MediSave Account, actually.)
When you make a property pledge you're doing so with the intention to withdraw funds from your RA, yes. If you do this, you can do it as early as your 55th birthday or much later. Unless you need the funds immediately, it's generally wise to wait -- and (I would advise) not to make a property pledge at all, assuming you're not in financial distress. As I've illustrated in calculations posted previously, RA and SA savings are quite attractive primarily because of their above market interest rate (4% floor rate currently). That's something to savor and enjoy as long as you can, if you can.