When activating an Apple Watch eSIM, the process is usually pretty straightforward. Most providers are using a QR code that you can scan during setup – this transfers the eSIM data and off you go. Not with O2 though – at least here in Germany; they open a builtin web browser and require you to log into your account to order the eSIM from within your account.
So I tried this. During eSIM setup in the O2 login form, you can choose between login via E-Mail-Address or phone number. If you try the E-Mail-Address you get an error message w/ "Please register with the O2 App". If you use the phone number here, you'll get a prompt "This device's phone number does not belong to your account. Please use a different account". And now you're stuck.
So after investigating several ways (the O2 support not being helpful at all…), I found the culprit: When you have multiple tarrifs in your account, the automated process does not scan all your phone numbers, but only takes the first one, the "main phone number". What you now have to do is to login (best via web portal) and change the main phone number to the one you want to order an eSIM for (you can change it back again after the process).
Once you did that, you can finally use that phone number to login during the eSIM setup process and ordering should work.