The 20-year-old, who had been at West Ham since the age of 10 and signed his first professional contract in 2010, has the option of a third year at Boundary Park.
He will be available for the Latics' opening fixture against MK Dons as his paperwork was registered in time.
Montano, who can play as a striker or on the wing, spent loan spells at Notts County, Swindon, Dagenham and Oxford last season, scoring 10 goals in 39 appearences.
Dickov believes his new signing can provide his Oldham team with a variety of attacking options this season.
The Colombian joined the Latics after training at Boundary Park during pre-season.
Montano received interest from clubs in Italy and France but opted for Greater Manchester instead - a decision Dickov puts down to the 20-year-old's desire to play first-team football.
"It wasn't so much Cristian being on trial, but him coming in and getting a feel for the club and for us to do likewise," Dickov told the club's official website.
"Cristian is another piece of the jigsaw and a left-sided player who is quick and talented.
"He is left footed, but can play on either wing and he loves to take on opponents and to get in crosses and shots. He can also play up front so he gives us lots of options.
"I gather he could have stayed at West Ham, but he was keen to move because of his wish to play football rather than being a squad player."
