Off-Peak Oyster single fares apply from 09:30-16:00 and after 19:00 Monday to Friday (based on when you touch in, not when your service leaves).
If you travel into Zone 1 on Tube, DLR, London Overground or some National Rail services between 16:00 and 19:00 on Mondays to Fridays, you will be charged an off peak fare.