He suspected his combination of business and local government experience had given him the edge.
Among his priorities, if elected, would be helping to push through reforms of the Resource Management Act.
National was not taking the byelection lightly and would "throw everything into the campaign," he said.
In the March 28 poll Mr Osborne will go up against NZ First leader Winston Peters, Labour's Willow-Jean Prime, Mana's Rueben Taipari Porter and Act's Robin Grieve. The Green and Maori parties are not standing.
The byelection was triggered by the sudden resignation, for "personal reasons", of Northland MP Mike Sabin.
Mr Osborne has previously managed Kaitaia's Te Ahu Centre and co-owns Doubtless Beauty with his wife Jodi. They live in Taipa and have two children.
Mr McCallum was believed to have been favoured by the National Party caucus as Mr Sabin's replacement, but is not well known in the Far North where most of the delegates are based.
Ms Rolleston is believed to have performed well at selection meetings but counting against her was the fact she lives outside the electorate in Helensville.