Soldato
If its anything like the OP3 you could only get them on PAYG for a very short time, then you had to buy them on a contract.
The good news is that you can use the O2 refresh program, which is what I did, you get the phone on the pay nothing, lowest contract deal you can get, and ask to do it on a "Refresh" contract. This separates the 0% finance for the phone from the contract, and as soon as you pay off the phone then the contract lapses.
And you can pay the phone off the same day if you want, then either get rid of the contract completely or upgrade it to a cheap SIM only deal with O2 to use as your second SIM.
Eh? I three weeks ago got an OP3 on PAYG from O2...