I have seen quite a number of times the date of 1096 floating around, but as far as I know that is the earliest recorded date of soneone teaching in the city of Oxford.
If that standard is used for Valladolid, then the founding date could be stated as 1085, but nobody would consider that. The generally agreed upon date is 1243, when "major schools" were already organised there. If one wants to be purist, the date to be considered should be 1346, when the pope granted the title of "studium generale" to the "major schools" that were sponsored by the Crown and the municipality.
Considering this, what about Oxford?
There are some previous answers about this that should be helpful:
Oxford University was established c. 1096. How have applications and admission processes changed throughout these past 900+ years?, by u/sungainstgold
What would a day at the University of Oxford look like back in 1096? by u/mikedash and myself