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
Fluamao Cliffs completed
Rode the Fifth Legendary Animal
Pulyang Heights completed
Stomped a Lapicrab 5 times
Kept an Astendeon airborne for 10 seconds
Yeccaph Mesa completed
Unkh-Tuur Desert completed
Cavulta Canyon completed
Rode the Fourth Legendary Animal
Gathered 3 Caloobs around you at once
Lake Ober completed
Surfed with Scipuins for 30 seconds
Rode the Third Legendary Animal
Sag'Ya Valley completed
Malaha Plain completed
Rode the Second Legendary Animal
Agneya Plateau completed
Walked through water for 20 seconds
Auqui Pit completed
Rode the First Legendary Animal