Should we transfer all of our OA to SA, use HDB loan for our BTO & cash payment for the monthly loan?

I wouldn’t transfer all the OA to SA. Would transfer some, depending on comfort level, but not all. Safer to have some buffer in case of any job uncertainties.

A) OA to SA?

1) Money transferred from OA to SA is one-way, you cannot reverse it later.

2) Are you sure of the future that there are no need for your OA for housing purposes, education loans (not just for you but your future kids) etc

For that reason, I prefer not to transfer OA to SA.

4% return can be obtained relatively easier with long term stocks or funds.

B) To use cash or CPF-OA is another discussion.

Some may say cash is better because CPF-OA you are required to pay the interest plus 2.5% back to yourself.

I don't see it as negative and I will prefer CPF because CPF money is very difficult to use, and cash is more precious for liquidity and easy available for other opportunities.

C) HDB loans or bank loans.

No doubts, bank loans. Lower than the current 2.6% HDB loan and quite reasonably sure that it will be below 2.6% for the decent future.

Given your loan is the largest at the beginning few years, you can be sure bank loans will have the lowest expense compared to HDB loans.