The 35-year-old midfielder was spotted in the stands at Latics’ pre-season friendly with Bury and has now penned a one-year contract.
Former England international Dunn, who left Blackburn at the end of the season, told Oldham’s official website: “I’m really looking forward to joining up with the rest of the lads and putting my boots back on.
“I’ve been fortunate enough to watch a few games during pre-season so there is a lot of good, young players so hopefully with my experience I can bring them on as well.
“From what I’ve seen so far, we seem to pass the ball really well and I’m sure I can fit into that really well.
“Hopefully I can chip in with assists and some goals as well. I’ve got a good record and I’ve scored a lot of goals from midfield in the past so we’ll see what happens.
“I need to do a bit of fitness work in the next few weeks but then I’ll crack on and start showing the fans what I’m all about.”