What do you all think about using CPF OA to pay for your HDB fully?

Honestly for most young couples, using OA fully makes sense in the early years when cash is tight. Wedding, renovation, a baby, everything hits at once. Preserving cash liquidity when you're financially stretched is more important than optimising CPF interest. That's the practical reality, textbook optimisation assumes you have surplus cash, most don't in their early 30s. What I did, use OA fully at first, then once our income grew and cash freed up, we started paying partly in cash and even made voluntary capital repayments. So it evolves with your life stage. Don't stress about the accrued interest bogeyman, just don't over-borrow and you'll be fine.

I'm firmly in the pay-some-cash camp, and here's the maths. Your CPF OA earns 2.5% guaranteed. The HDB concessionary loan charges 2.6%. So using OA to pay the flat isn't free, you're forgoing 2.5% growth on that money, plus the accrued interest you must return to your CPF when you sell keeps growing. Many people get a nasty shock at sale time when they realise the CPF plus accrued interest eats most of their proceeds. If you have spare cash, servicing part in cash preserves your OA compounding for retirement. That said, if paying cash means you have no emergency buffer, then use OA, liquidity trumps optimisation. It's a personal balance.