Let's do the math for a 6 month T-bill yielding 3.32%...
Scenario 1: Leave Dollars in OA
$983.45 * 2.5% * 6/12 = $12.29 interest
Scenario 2: Buy the 3.32% 6 Month T-Bill
Interest = $16.55
Lost Interest (A) = $1,000 * 2.5% * 1/12 (September, 2022) = $2.08
Lost Interest (B) = $983.45 * 2.5% * 1/12 (March, 2023) = $2.05
Net Interest = $16.55 - $2.08 - $2.05 = $12.42
Difference (in favor of Scenario 2): $0.13
....But there are some footnotes. The bank (DBS/POSB, OCBC, or UOB) charges fees for the CPF Investment Account. So this math doesn't work at small dollar values of T-bills such as this ~$1,000 example because the fees will swamp the 13 cents of additional interest. Also, I've implicitly assumed that there are no other inflows into OA. However, if there are well-timed inflows then you might not lose as much CPF OA interest. Also, if you're able to roll T-bills over to the next auction, within the same calendar month, then that'll effectively reduce the interest loss. And finally a 12 month T-bill or other longer tenor SGS can work better.