Here is the full list of all 21 Omno achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
Finished Omno with all areas completed
Pulyang Heights completed
Kept an Astendeon airborne for 10 seconds
Gathered 3 Caloobs around you at once
Surfed with Scipuins for 30 seconds
Agneya Plateau completed
Auqui Pit completed
Walked through water for 20 seconds
Fluamao Cliffs completed
Rode the Fifth Legendary Animal
Yeccaph Mesa completed
Unkh-Tuur Desert completed
Rode the Fourth Legendary Animal
Cavulta Canyon completed
Lake Ober completed
Rode the Third Legendary Animal
Sag'Ya Valley completed
Malaha Plain completed
Rode the Second Legendary Animal
Stomped a Lapicrab 5 times
Rode the First Legendary Animal