He started and ended his career at Viking, before going on to play in the Premier League for Southampton, Blackburn Rovers and Manchester City.

He played a total of 280 games for the club, scoring 88 goals, before moving to Southampton in the FA Premier League in October 1996.

In only his third game for the Saints he opened his goal-scoring account in spectacular fashion with a hat-trick in the 6–3 win against Manchester United on 26 October 1996.

Østenstad was now paired with Kevin Davies in attack but failed to score until a substitute appearance against West Ham United on 4 October 1997.

Østenstad was part of the Norway squad for the 1998 World Cup held in France, but was given little chance to make any impact.

In the 1998 close season, Kevin Davies departed for Blackburn Rovers, with James Beattie joining the Saints as part of the deal.

11 days later, Dave Jones decided to move him on and arranged an exchange deal with Blackburn Rovers with Kevin Davies returning to The Dell.

Despite being re-united with manager Graeme Souness and dropping down a division, Østenstad struggled to make an impact at Ewood Park.

He started well, scoring a brace in his second game, in a 2–0 victory at Norwich City on 28 August 1999 and completed the 1999–2000 season with eight league goals from 28 appearances, playing alongside Ashley Ward and Lee Carsley.

Rovers won the 2001–02 Football League Cup and despite being left out of the squad for the final, Østenstad did appear in some of the earlier rounds.

[9] Østenstad made his debut for the Norway national team in 1993, coming on as a substitute to score twice against the Faroe Islands.