All ways to top up here Guide How to Top Up (PAYG)
There used to be a restriction on how many unique phone numbers on O2 PAYG could be topped up from the same credit card. With 5 x PAYG phones in our household, we have 3 different cards used for top-ups (2 on one card, 2 on another and one on the third) - all works well.
Also be sure, if using mobile browser, to type in the card number - that 'use your camera to scan your card' feature does not work...
Good luck, @Omar .