O’Brien, 20, has been elevated under the AFL’s mid-season rookie rule, which allows eligible clubs to add a rookie to their senior squad without placing a player on the long-term injury list.
The 201cm, 98kg, ruckman was temporarily upgraded to Adelaide’s senior list on two occasions last season under the same rule, and also to help cover injury.
Crows Head of Football David Noble said the promotion was good reward for O’Brien.
“We’ve had a spot on our senior list available since Round 11 and Reilly has been the one to put his hand up for it,” Noble said.
“Reilly’s SANFL form has been consistent throughout the season and elevating him to our senior squad also gives us some added depth in that ruck area.”
O’Brien has played three NAB Challenge games, but is yet to make his AFL debut.
The competitive second-year Crow was Adelaide’s best player in Sunday’s loss to Glenelg.
A rookie must be on the senior list to be eligible for AFL selection, including as an emergency.