The young man, a Norwegian national, was previously known to police, Oslo police officer Andreas Strand told AFP, without specifying his role in the attack.
The suspect denied all accusations, according to Strand.
Nehma Ali Shah, the imam of the Central Jamaat Ahle-Sunnat mosque, was attacked with a "sharp object" by a masqued assailant close to his home on June 17.
The motive behind the attack remains unknown, while witnesses told police there was a single attacker.
Norwegian police had already arrested a Norwegian man of Pakistani origin on June 30 on suspicion of being either the perpetrator or an accomplice in the assault.
The Jamaat Ahle Sunnat mosque is reported to have been caught up in an internal power struggle, with some questioning the imam's leadership.
In 2006, several men attacked worshippers at the mosque during prayer time, injuring four.
Shah has repeatedly condemned religious extremists, and made a 2006 visit to the Oslo Synagogue after shots were fired at the building.
However he also attracted controversy last year when he claimed that Norwegian media were controlled by the Jews.
COMMENTS (4)
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They are taking their blood religious politics to abroad .
Norway govt. distribute 300 million dollars to all religious institutions, Pakistani imams and their hedquarter Ulema get share each year. Get more information. It is a money matter.
when pakistani muslims put "sec" on their mosques, that means they are encouraging sectarianism,the pakistani muslims in norways are pure case of indulging wealth and prosperity of that country and unwilling to adopt the secular culture of scandinavia.
Some Muslims bringing a violent mentality and culture to Norway. This will not fare well for the Norwegians. See Syria.