The midfielder joined from the Premier League strugglers in the summer and he helped Wrexham win 4-3 on penalties after the third round clash ended in a 3-3 draw.
The Reds’ reward is a fourth round date with fellow Championship outfit Ipswich Town at The Racecourse on Friday night and O’Brien has set his sights on securing a place in the last 16.
“It was kind of crazy, a weird draw to get against Forest for me, but I definitely enjoyed the win,” he said.
“It was nice to get one over them.
“The FA Cup is such a big cup with a lot of history and it is nice to be able to play in it.
“Hopefully we can progress against Ipswich.”
Ahead of the Forest clash, the Wrexham players watched footage of the club’s famous giant-killing acts and O’Brien says it provided extra motivation to write their own cup headlines.
“When we got shown the videos of who Wrexham have beat, you want to do the fans and the team proud because we are only here for a short amount of time,” he said.
“The people before us have done what they’ve done and hopefully we can leave our mark as well.
“It was good to watch them back.”
Wrexham, still sixth in the table despite Saturday’s 2-0 defeat to play-off rivals Millwall, also host third placed Ipswich in the league the following weekend.
“It’s good to play them twice because you can kind of feel each other out,” added O’Brien.
“They will be cagey matches because I am sure Ipswich will want to win in the FA Cup as well as the league.”