The 25-year-old, who has agreed to a two-year deal with the option for another 12 months, spent pre-season training with Latics while attempting to resolve a contract dispute with his former club Swindon.
Rooney had agreed to join the Robins on a permanent basis this summer but, when they were taken over in February, the new owners reviewed the contract in light of its "high value".
That has subsequently opened the door for Oldham to snap up a man who scored 10 league goals at the County Ground last term on a free transfer.
Speaking to the club's official website, Latics boss Lee Johnson said: "It is a great acquisition, especially as there was a lot of interest from other clubs.
"Adam is a good bloke and a good character. He also understands the intricacies of how important it is to organise on the pitch.
"Adam is a good age, is mobile and has a good scoring record. He brings others into play and is not afraid to put his head in where it hurts and take a cut to the eye if that is needed.
"Adam is our fifth striker and we certainly have plenty of options up front."