You can just text the code as long as you took out the contract online or instore and it's within the first month.
text OPEN followed by your open code, mobile number, name and postcode to 2020
It can take 2-3 days for a number to register as an o2 number on the system after porting. Try calling customer service or attempt again in a couple of days. You have 30 days to get the code applied.
Track usage, upgrade and get support with the My O2 appDownload My O2