This happened to me some years ago, 3 transactions for nearly £4,000.00 on my credit card to 3 well known companies.
I simply had to sign a letter to say I had not made these purchases and the credit card company sorted it out within a week.
I have no idea how it was done and it's really of no interest to me.
The card had never left my sight at any time.
In your case O2 just happen to be the company where the money was spent, they can't help you.
Best left to Barclaycard to sort it out.
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