To elaborate on an earlier comment, let’s suppose the BTO costs $300,000, you’re going to use OA for the 10% down payment ($30,000), and you want to keep $30,000 in OA ($15,000 per person) when you pick up the keys next year. So you need $60,000 of OA total, but the $60,000 target is your aim point for next year. What you can/should do now, with that sort of plan (if that’s your plan, or similar), is figure out how much OA to transfer into SA before the end of this month, factoring in expected compulsory contributions and 2021 interest credited between now and key pickup to your OAs. That is to say you can get a jump start, right now, on earning the higher SA interest on some portion of what’s currently sitting in OA. If you wait until just before key pickup then make the transfer, you’ll miss several months of SA interest you could already be earning on that surplus.
Does that make sense?