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