I'm certainly not the poster to this thread who is refusing to consider even the possibility of not using OA for housing!
Assuming the assumptions I've carefully explained, I would point out this important reality. In order to beat SA's 4% you would have to be reasonably well convinced that you could plow OA dollars (that you need as OA, in order to afford a down payment for example because you don't have enough cash and remaining OA) into a real estate purchase that would reliably generate a total long-term return (net of all costs, including taxes) rather well above 4%. I say "rather well above 4%" because SA's 4% interest rate is available immediately, backdated to the first of the month. For example, it's December 6, 2018, as I write this. If you transfer OA funds to SA today, or even on December 31, then the higher 4% interest rate starts from December 1, 2018 (the whole month of December). If you leave OA funds as OA to wait to build up enough funds for a down payment (because you'd be short of other sources of funds for a down payment), then you're running behind (2.5% v. 4%) for however long that build-up takes. So, after you make your real estate buy, you have to blow past 4%, reliably and net of all costs, in order to make the real estate investment end up better than SA. You've also got to account for the higher non-CPF retirement savings that you must have in order to account correctly for the lost SA 4% opportunity on your savings. And you have to account for the "lumpiness" of real estate. Surplus SA dollars can be withdrawn one dollar at time. That's not true of a whole property. You have to sell the whole thing to liberate funds, and then what? Where do you put those dollars to earn 4%? Tough.
Does this financial math seem realistic to you? Do you think you can reliably hit a ~4.5% or better total return on your real estate tycoonism (net of all costs), and every OA dollar is absolutely critical to achieving that investment goal? If that's how you want to double down on your financial bets, you can. I don't think it's wise (with the key assumptions I've described), but it's up to you.
I happen to think that sort of ~4.5% or better total real estate return forecast ranges somewhere between unlikely and delusional. Why would/should total real estate returns outpace general inflation for decades by ~300 basis points or more? Does that make any sense? Is that logical? But if I'm wrong, no big problem. The real estate you already own (and continue to own more of as you pay off your mortgage on schedule) will do just fine, and your SA will lag only a little behind the wonderful, immense riches you think you'll achieve in Singapore real estate. You'll also only be a bit late to this raging, multi-decade bender of party, because you're not allowed to transfer OA funds into SA once your SA hits the Full Retirement Sum. Thereafter, OA dollars start piling up again. Who knows, you might even catch a real estate cycle at its trough instead of its peak.
Some people might argue that it's wonderful to have some level of persistent mortgage debt, i.e. leverage. Maybe, until it isn't. There's a risk in that, and there's a cost to defend against that risk. I have no problem with manageable and low cost debt, but it's the unmanageable and/or high cost debt that can really bite you hard.
Anyway, to repeat, you only need OA dollars as OA if they are vital to your housing needs or any punts you want to take in real estate. If you're reasonably (or more) flush with funds and simply don't ever need OA as OA -- like us, like many -- then the smart play is to push OA into SA. Because you can already do everything you could ever possibly want to do involving real estate without OA as OA. Some of us are that fortunate, and for the increasing numbers of those who are so fortunate, CPF can be extra wonderful.