Jim Rosenthal has resigned from Oxford's board of directors.
The sports television presenter, 64, was invited to join the League Two club as a director two years ago by close friend and chairman Kelvin Thomas.
But the avid U's supporter has now stepped down with immediate effect, just two months after Thomas himself opted to depart.
Owner Ian Lenagan took over from Thomas in July and he along with his two sons now remain as the only directors on Oxford's board.
Rosenthal told the Oxford Mail: "I have felt it necessary to resign from the board of OUFC with immediate effect. I don't believe it is in the interest of the club to air my reasons at this time.
"I offer my continued support to (manager) Chris Wilder and the team and will support home and away as I have done for more than 50 years. I will not be making any further comment at this stage."
Oxford are not currently commenting on the matter.