Monthly Limit Reached for O2 Charge to Mobile on App Store

if you are a customer of O2 as your carrier then I would probably attempt to contact them to hear what was up

if you are not a customer of o2 then I would view it as an attempt to

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

of cause even if you are a customer of o2 it could still be phishing which is why you should contact them directly and never by links in such sms

If you receive a text claiming that you’ve reached your monthly O2 charge limit, do not click on any links or share personal details. Instead, log in directly to the official O2 Mobile app to verify your account status. To be absolutely sure, you can also contact O2 Customer Service for confirmation. This way, you avoid falling victim to phishing attempts and ensure your information stays secure.

I agree with the other responses. Make sure the message did actually come from O2. If it did not, you need to change your privacy settings with O2, as you may have agreed to receive messages from 02 partners, which means they sold your contact information to possibly less than honest or credible companies.