Not a straightforward answer. I would priortise the following use of money in CPF OA in this order:
1. Payment of housing loan
2. Holding 2.5% cash in anticipation of any housing related payment
3. Investment in STI through CPFIS
4. Earn 2.5% for liquidity consideration OR put in CPF SA
I think 2.5% "risk free" is a terrible option for those with a long investment horizon. The money is locked in anyway, if you have no need for it for housing why not just invest it in a diversified equity ETF (even though STI is trash at diversification).
I have finally put in all my CPF OA in STI ETF, and yes I pay $5.35 a quarter for charges on my $60+k in STI, but at least my money is not earning a paltry 2.5%, with the excess returns made by GIC given to the fund our Budget. I can sleep better knowing my money is put to real use (ensuring retirement adequacy), rather than structured in a way that is not in line with my own goals.
I am of course happy to slowly liquidate this higher risk ETF to purchase something globally diversified with a reasonable TER, or put it back in CPF OA for the 2.5% (if still available) at older age.