How do I know I have fully utilized the ocbc 365 credit card for cashback? Any tip or hack to save while earning the $80 cashback rebate? ?

First, if you are only spending $800/month on the OCBC365 card it is not possible at all to hit the cashback cap of $80 (which is a 10% cashback) since the OCBC365 rebate categories only give 3-6% cashback. You should think of the $80 as the upper limit on what OCBC will give, which is really only relevant if you spend much more than 800/month on the card.

Based on what you shared and assuming you spend ~800/month each month, for May you achieved 18.25/800 = 2.3% overall cashback, which I would say is good enough to keep using the card. However what you got in April and March is very poor and if those are typical months you may want to consider other OCBC cards such as the OCBC Titanium Rewards or FRANK credit card that do not have minimum spending requirements. Finally, the 0.3% bonus interest rate on OCBC360 account is not considered partof the $80 cashback.

One tip I could share to help maximise your cash rebate is to do a quick search on https://www.whatcard.sg/ to see if you get bonus cashback on OCBC365 card before spending. Full disclosure: I am part of the team that developed WhatCard to help ourselves figure out how to best maximise credit card rewards.