On Saturday O'Brien made his first start for the Rams since suffering a cruciate ligament injury last December and helped them to a 2-0 win over Bristol City.
He told the club's official website: "It feels brilliant to be back. During the week I didn't know what was going to be happen, I was hoping that I would get a chance and for me to see my name in the starting XI was fantastic.
"It was great just to get the win but the clean sheet makes it that extra bit special. The lads were unbelievable and you cannot take anything away from them - everyone played their part.
"I had a bit of cramp towards the end of the game but aside from that I am on a buzz now coming back into the side and hopefully I can push on from here."