Open MyO2 for the account you want to be the lead device
http://www.o2.co.uk/myo2Then select "Bring in other products" from the right side of the page.
You will be asked to sign in with the other accounts you want to merge.
Note, this cannot be undone so make sure you want them all linked forever.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)