It's not just the M25 thats a problem its getting to it.
Depending on where you are in Essex, the A12 regularly backs up the length of the Brentwood Bypass back to the Shenfield turnoff, the A127 out of Southend is usually a nightmare, the A13 has roadworks on the Stanford-le-hope bypass and the M11 is very busy.
If you are further out, ie.Colochester or beyond, you will know how busy the A12 is down to Chelmsford
If it is vital that you are in Oxford by 10.30 the suggestion that you leave at 5.00/5.30 is sound advice