3G ending isn't too complex, SMS and voice still drop back to 2G, data might not work well (or at all) if you presently use 3G for data, data will work fine if you already use 4G data. The looming, but date not fully set-yet 2G "switch off" might not be that simple, data will work (if it works on 4g already) but with 2G gone voice calls and SMS have to go over a new "standard" called VoLTE, only it isn't quite standardised. Some device models will have a VoLTE "standard" that isn't the same standard that some networks will have. I don't know whether there is any way of testing whether your specific phone's VoLTE version matches the versions which O2's network expects.
