My mum is 61 this year. She has a substantial amount in OA and much lesser in SA and the FRS in RA. Can I check if she can withdraw funds from her OA account as and when she likes now (based on withdrawal after age 55)? Any implications other than interest lost from OA?
She can withdraw funds at any time, as often as she wants, in any increment up to her balances (less her retirement and Medisave set asides). Unfortunately the first dollars she withdraws must come from her higher yielding Special Account. Then, when her Special Account is drained, her withdrawals will pull from her Ordinary Account.
Logically that makes sense from the government's point of view. They don't want to be
too generous, so withdrawals come from the higher yielding funds first.
There's a CPF "hack" available to work around this behavior, but it only really works if your mother wants to make a single, fairly sizable withdrawal as a one-time event (basically) and will then allow her Special Account funds to keep earning interest for at least a while. The hack involves using the CPF Investment Scheme (Special Account) to buy a safe outside investment. A Singapore T-Bill works well for this purpose. With most of the Special Account funds tucked away in the T-Bill, withdrawals then come predominantly from the Ordinary Account. Then you return the funds from the T-Bill to the Special Account. If you time this very well and get a good T-Bill offer on the secondary market (of a T-Bill maturing "soon"), you don't lose too much Special Account interest. But it's a tough hack to pull off, really.
Can she also transfer her OA to SA to earn higher interest and still retain flexibility to withdraw the funds if the above is possible?
No, I'm afraid that opportunity ended on her 55th birthday. And it was only an option as long as her Special Account was below the then current Full Retirement Sum.
I'm thinking of the viability of topping up her SA for tax rebate, before withdrawing from her OA in lieu of household allowance.
Not viable at all. If you (or anybody) wants to top up her account(s), you only have three choices at this point: Medisave, Retirement Account, or "all three."
1. Anybody can top up her Medisave Account if she has not reached the current Basic Healthcare Sum (BHS) already and as long as that top-up fits within her (not the giver's) CPF Annual Limit. (If the top-up exceeds her CPF Annual Limit, it will be refunded back to the giver, without interest.) She would qualify for tax relief on her own tax return for that top-up, if she's eligible, but nobody else can.
2. Anybody can top up her Retirement Account if she has not reached the current Enhanced Retirement Sum (ERS). However, since she has already reached the Full Retirement Sum (FRS), nobody gets any tax relief for that RA top-up.
3. Anybody can top up her accounts using an "All Three" top-up. This type of top-up does not qualify for tax relief, for anyone. It must fit within her CPF Annual Limit ($37,740). Exactly how those funds flow into her various accounts is a little complicated, but they are allocated (and spill over, if applicable) exactly the same way compulsory contributions from employment would flow.
I think that covers all her top-up options.