A person familiar with the situation told The Associated Press that McDaniels is expected to serve as an offensive assistant under O'Brien for the rest of this season. The person spoke on condition of anonymity because the team hasn't made an announcement.
The move was first reported by ESPN.com.
O'Brien said at his news conference Saturday he intends to remain the Patriots' offensive coordinator as long as they stay in the playoffs.
"I'm sure Josh will have opportunities around the NFL to possibly be a coordinator or better throughout this process," Kevin Demoff, the Rams vice president and chief operating officer, said at the time. "It's going to be fluid, but we'll figure out what's best for both parties."
The Rams scored the fewest points per game in the league, 12.1, gained the second fewest yards, 283.6, and the third fewest yards passing, 179.4. Sam Bradford struggled at quarterback with just six touchdown passes and six interceptions in McDaniels' system, which had many more longer developing pass plays than the Rams had in 2010 under Pat Shurmur.
McDaniels said on Dec. 29, three days before a season-ending 34-27 loss to the San Francisco 49ers, that he had no regrets.
"You're blessed to have the opportunity to coach in the National Football League," he said. "You have to do everything you can to try to help the team win."
In that season, Tom Brady set a single-season league record with 50 touchdown passes for an offense that averaged a league-best 411.2 yards. It also was first in yards passing, 295.7 and points per game, 36.8.
Now McDaniels is poised to be reunited with Brady and another prolific offense in time for the Patriots practices for their divisional playoff game next Saturday. As the top-seeded team in the AFC, they earned home-field advantage throughout the conference playoffs.
The Patriots were second in the league in overall yards with 428 per game and yards passing with 317.8. Their average of 32.1 points was third.
"It's a group of individuals that has to become a team and has to work together, has to fit together the right way," he said. "Every time that I've switched venues, I've learned that in a different way. I went from New England to Denver and learned that once, and then from Denver here and learned it again.
"I think it's about how people interact and people mesh together and you have to continue to work in this business to make sure that you have the right group of guys together."