He cannot transfer his OA dollars directly to his RA. He would have to transfer his SA dollars first, then he could transfer OA dollars. So this is possible (assuming he transfers half his OA — that amount is flexible):
Before
OA: $100K
SA: $30K
RA: $40K
After
OA: $50K
SA: (zero)
RA: $120K
In this example he would substantially improve the amount of interest he earns (since $50K would be upgraded from 2.5% to 4.0% interest — that's an extra $750 of interest just in the first year), and he would substantially boost his monthly retirement income. (If he's enrolled in CPF LIFE his monthly retirement income would substantially increase for the rest of his life.) Presumably he cannot withdraw much (or at all) today from his SA/OA since he hasn't funded his RA to at least the Basic Retirement Sum (with property pledge/charge), so that particular problem would be fixed as well.
There might be a better option available, though, depending on his family situation. Let's suppose he's married and his wife's accounts look like this:
Wife's Accounts
OA: $80K
SA: $40K
RA: $100K
In this situation his wife can transfer her $80K of OA to his RA, and he can return the favor by transferring $80K of his OA to her RA. That's all without touching their SAs.
And there's yet another option, but it's awkward. Technically he could invest up to $80K of his OA in the CPF Investment Scheme in something safe and simple, like a Singapore Government Security due to mature soon. Then he can close his CPF Investment Account (before the SGS matures), whereupon the SGS can be transferred to his CDP account. Finally, he can use that cash to top up his RA. It's a "backdoor" way to withdraw funds from his OA without touching his SA. Like I wrote, it's awkward. It requires careful planning, execution, and some knowledge.
....And there's yet another option, not mutually exclusive. If you (his son) qualifies for tax relief when you deposit $8,000 into his RA, then you should do that. Even if he hands you the money to do it. And then you can split your tax savings however you wish. Let's suppose for example you're in the 7% tax bracket, so you would save $560 (7% of $8,000) on your income tax next year. Then (for example) he could hand you $7,500, you deposit $8,000 into his RA ($500 more), and you keep the $560 of tax savings next year. Everyone wins except IRAS. Other siblings and other eligible family members who can benefit from tax relief can play this game, too.
