Per current rules the HDB concessionary loan rate is the OA rate plus 10 basis points. There'd be screams in agony if that HDB loan rate increased to 4.0% for example (the recent 3.9% T-bill rate plus 10 basis points). So I don't think a general OA rate increase is likely with the HDB loan rate linkage, although the OA rate peg to "Big 3" board rates doesn't seem sensible either. The banks obviously just leave their board rates alone while making their promotional rates their everyday rates. (They play these games with fixed deposit rate linked home loans, too.)
I think it's time for some fundamental rethinking of OA. I understand and mostly appreciate the government's desire to "encourage" Singaporeans to own homes. (Although Switzerland seems to manage quite well with much lower rates of home ownership combined with impressive household wealth accumulation.) So that means saving for a down payment and servicing a mortgage. But is OA as currently constructed the best way to do this? I have my doubts. It seems quite obvious that Singaporeans (some) are sitting on vast quantities of OA dollars earning a now fairly unattractive 2.5% interest rate. And they understandably want to find other, more productive outlets for these balances. So they're chasing real estate (even when they probably shouldn't), and that's got to be putting some upward pricing pressure on housing in Singapore. Force people to save for something even when they don't need to and you'll get some price inflation on that something.
I think the CPF Advisory Panel had/has the right idea with the LRIS. It's very strange that the CPF Board hasn't adopted the Advisory Panel's LRIS recommendations by now. Beyond that, what about refocusing OA on first-time home buyers? Even barring OA dollars from second and subsequent homes? How about making the employee's share of OA contributions voluntary (though still eligible for tax relief) once the employee has reached a certain CPF savings and/or housing threshold? I don't have firm opinions on these questions, but I think the questions ought to be asked and carefully considered since there do seem to be some problems particularly with the OA leg of CPF.