You may only apply for a supplementary card when the principal card is approved.

After I re-re-recheck with a few CS, looks like OCBC Ti doesn't seems as good as it is.
Some Auto debit will have a different MCC, so it is not guaranteed that you will get the 5%. Swiping through merchant terminal is 100%. However, according to CS, so far there is no problem with utilities bills via autopay but there are exception. Some online payment may have different MCC.

For petrol station, 5% rebate only applicable for petrol and not purchase made at the corresponding shop/mart.

For groceries, so far only Giant is confirmed. Others like Tesco, Jusco, C4 may consider under departmental store MCC. You may not guaranteed to get rebate from it. The CS told me to try on these shops, and you will know from the next statement.

For dining, only restaurant (most of it) are eligible. But you will not get 5% rebate for cafe like Starbucks, coffee bean and so on.

For those who are getting the first statement this month, do share with us which merchant is entitled for 5%.

All I can say is Good Luck everyone!