Cox has signed a one-year deal with an option for a further year.
Cox was involved with the Reds last season and made one start and three appearances as a substitute after they acquired the defender from Central Midlands League outfit Swanwick Pentrich Road on dual registration.
However, he now joins the Reds with a view to moving out on loan to gain valuable minutes and experience.
Reds boss John McDermott was pleased to have re-signed Cox: “I’m very pleased to have re-signed Oli, he has been excellent so far, he is training with us and he played very well against the Forest Under-23’s on Saturday. He came into the game well and defended well especially in the second half.”
“We had lots of triallists out again on Saturday but I felt he handled the game well and soaked up pressure in the second half. I was happy with our overall performance, we restricted them to limited chances and created plenty ourselves despite the result.
“Oli will move out on loan to gain experience which will be important for him”.