Here is the full list of all 37 OmniBus achievements.
Get 25 hats
Get bronze medals in all story missions
Complete World 5
Get 10 hats
Beat a level with a pushy bus
Complete World 4
Beat a level with a bus from the farm
Beat a level with a wooden bus
Complete World 3
Complete 3-5 after hitting a bumper
Beat a level with Gravity Bus
Complete World 2
Keep your hands and feet inside the vehicle at all times
Beat a level with Sir Bus
Beat a level with Ultra Bus
Complete World 1
Beat a level with Mr. Bus
Get silver medals in all story missions
Get gold medals in all story missions
Beat 1-4 with one trick
Go through the goal posts in 2-2
Only break two banks in 2-Star
Make sure everyone dropped off in 3-6 lands in the wedding zone
Complete 4-2 with at least 17 cows
Don't get hit by any attacks in 4-7
Collect your 40th chicken
Knock off a giant hat
Get at least $100,000 in Free Play
Get at least $500,000 in Free Play
Get at least $1,000,000 in Free Play
Get all the hats
Unlock a special bus
Get gold on all levels and get every hat