If I were to have cash on hand ($30k) and cpf-oa @ 2.5% ($30k) and am taking up a bank loan at 1.5%, would it be advisable for me to clear my housing loan or let the cpf remain in OA or invest in ?

I prefer to clear your housing loan first as before you can even invest, you should clear up any loans and debts you currently have.

Are you late 30s or late 50s? When do you anticipate retiring, and not having a stable income? If you are only late 30s (like i am), and have a healthy work attitude (aka will continue trying to make money till 60s/70s), then i would say you should NOT clear the housing loan, and use the cash to go invest (this is assuming you have an emergency fund stashed away or you have a (very) stable job).