Question

How do you get to Route 120 in Omega Ruby?

Answer 1

It's east of Fortree City. If you want to get to Route 121, you need to get the Devon scope from Steven, catch or defeat the Kecleon, use the Devon scope on the Kecleon in front of Fortree gym, and defeat Winona. After defeating Winona, you should be able to access all of Route 120.
source

Answer 2

To get to route 120, you have to get through Fortree city. Climb up a ladder (I think there are 2 but I'm pretty sure they both take you to route 120. If not, just try the other ladder) and walk on the tree houses and rope bridges for as long as they will take you. At the end of the chain of tree houses, there is another ladder, so climb down it. Depending on where you get down from the ladder at, there might be a curve that you have to go around, but you should see the entrance to route 120 from there.