Hi and welcome.
Occasionally online deals can be a little better.
If the allowances are better or the same with the same phone with the lesser monthly tariff cost ,then either print a copy off your computer and go into the O2 shop to see if they will match it ,or phone O2 on 202 from an O2 powered phone or from landline 0844 809 0202 > 5p / min
Do not use the Online Live Chat Facility for this.
If they can't / won't then you have 7 working days from when you opened the contract as a cooling off period.
Return the phone to the O2 shop and cancel.
Then do the contract online to take up the deal you have seen.