Østberg competed in his first rally in 2004. His WRC debut came at the 2006 Swedish Rally, driving a Subaru Impreza WRC 2003, where he finished 31st. He contested two further rounds that season, retiring in Finland and finishing 23rd in Wales. In 2007 he joined the Prodrive-supported Adapta team, competing in six WRC rounds with a more current Impreza. He took his first WRC stage win on a super special stage of the 2007 Swedish Rally and scored his first WRC championship point at the 2007 Rally Finland. He also won the Norwegian Rally Championship that year.
Through 2008 and 2009 he continued with Adapta Motorsport, scoring points on three rounds in 2009 with a best finish of sixth at the Rally de Portugal. In 2010 he mixed a Ford Fiesta S2000 with Subaru machinery across four WRC rounds and matched his eleventh-place championship standing from the previous year.
For 2011 Østberg moved to the Stobart M-Sport Ford team, driving a Ford Fiesta RS WRC. He led much of the 2011 Swedish Rally before finishing second, and also recorded second place at the 2011 Rally GB, ending the season sixth in the drivers' championship.
In 2012 he reverted to his Adapta Motorsport entry but retained the Fiesta RS WRC. He won his first rally at the 2012 Rally de Portugal after Mikko Hirvonen was disqualified — becoming the first non-French or Finnish driver to win a WRC event since 2005 and only the second Norwegian in history to win a world rally. He followed that result with third in Argentina and finished fourth overall in the 2012 championship.
Østberg returned to the Qatar M-Sport team for 2013 and secured two podiums before a move to the Citroën works team for 2014, alongside Kris Meeke. At Citroën he reached second place as his best individual result in 2014 and claimed fourth in the championship. In 2015 he was again fourth overall — described as the best of the rest behind the three Volkswagen works drivers — scoring two second places in the early part of the season.
For 2016 Østberg returned to M-Sport, this time driving the Evolution version of the Fiesta RS WRC alongside new co-driver Ola Fløene. He took two third-place finishes and ended the season seventh.
Østberg scaled back his programme from 2017 onward, sharing a private entry with Martin Prokop as he and his wife Beate welcomed their first child in February of that year. He continued to appear selectively in WRC rounds in subsequent seasons, remaining competitive at the front of the field on selected events.
His sole WRC victory at the 2012 Rally de Portugal remains the cornerstone of his career record, and his consistency through half a decade of works-level competition established him as one of Scandinavia's most capable drivers of his generation.