When cycling in the dark, you must have white front and red rear lights as well as a red rear reflector. You can use flashing lights but if you are riding in areas without street lighting, it’s better to use a steady front lamp.
If you choose to wear a helmet make sure it fits correctly. It’s also a good idea to wear light-coloured, fluorescent or reflective clothing or accessories that help other road users to see you in daylight and poor light.
There is more information in the Highway Code (Rules 59 and 60). It is also worth remembering that Thames Valley Police carry out regular spot checks in Oxford - you may be stopped if your bike doesn’t meet the standards.