To cancel or deactivate your OCBC credit card in Singapore, you can fill up the OCBC “Services for Credit Card” form which you can find online in PDF. In that form, you will find a “Terminate my Credit Card” section. Fill it up, print it out, and you can either mail it to OCBC or bring it to your nearest OCBC branch.
Do remember to utilise all your OCBC rewards points, transfer your air miles, and pay your credit card bill in full before you terminate your credit card. If you still have other OCBC credit cards, your OCBC$ rewards points will pool and remain in your rewards account.