The Os have had a poor start to the season,and languish in 21st - just six points ahead of Shrewsbury Town, who occupy the second relegation spot.
Os chairman Firoz Kassam said: "Graham has done a good job and bought some good players, but results have gone against us and we need to make changes.
"Graham assumes an upstairs role, and Darren will take over the first team.
"I have an excellent working relationship with Graham and hope that we can find a way to utilise his experience and knowledge at the club.
"Darren takes charge for the next three weeks and I have every faith in him and David Oldfield, who will remain first-team coach."