Dirksen, whose deal will keep him at the Liberty Stadium until 2014, has scored eight tries in 24 games this season and established a reputation as one of the Welsh game's most lethal finishers.
Ospreys head coach Steve Tandy said: "Hanno has enjoyed a great season and has really shown what a talent he is."
He added: "If he continues to work hard and shows the attitude and desire to improve his all-round game that we've seen from him since he joined the group, he could have a very bright future with the Ospreys."
Dirksen, 21, hails from South Africa's Gauteng province but moved to the United States with his family in 2006 and played in a non-cap game for the US Eagles against Munster four years ago.
He began college studies in Wales during September 2009 and signed his first Ospreys contract in January last year.