27-08-2015 08:28 - edited 27-08-2015 08:35
Have a look at this help link
You can call O2 30 days before end of contract and inform them you want to switch to payg..
Edited to add info about PAYG Tariffs
on 27-08-2015 09:23
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)