Five-cap O'Brien, 28, counts Millwall – where he had a popular 10-year stint – Sunderland and Portsmouth as his former clubs and was offered fresh terms by the Fratton Park side this summer.
But he has instead opted on a two-year move to Shropshire to spearhead Steve Cotterill's attack at Shrewsbury, where he bolsters a forward line including Daniel Udoh, Ryan Bowman and Tom Bloxham.
London-born O'Brien came through the youth ranks at Millwall, for whom he scored 44 goals in 226 games in all competitions across six seasons.
He moved on to League One big-hitters Sunderland in June 2020, where O'Brien scored 11 goals in 52 appearances in total across one-and-a-half seasons. He joins up with former Black Cats team-mate Tom Flanagan.
The powerful and strong-running forward penned a short-term, six-month deal at Portsmouth this January. He scored five goals in 17 games for Danny Cowley's side in the second half of the League One season – including at Shrewsbury in February.
Town supporters have been treated to an exciting raft of new additions this week as Cotterill has marked the squad's return to pre-season with four eye-catching new recruits, to make it five for the summer.
O'Brien adds to centre-half Chey Dunkley, previously of Sheffield Wednesday, Coventry loan wing-back Julien Dacosta, attack-minded midfielder Tom Bayliss from Preston and fellow midfielder Jordan Shipley from Coventry, who checked in at the beginning of the summer.
Shrewsbury's squad, which had looked light on numbers at the beginning of the week, has been beefed with new additions ahead of Saturday's pre-season flight to Spain – and the club have succeeded in attracting a number of good calibre lower league names.