The Oshkosh M-ATV’s fuel filler is at the fuel cap on the vehicle body , and in DayZ you need to face that cap with a filled canister or similar container to get the refuel prompt.

For DayZ vehicles in general, the fuel cap location can vary by vehicle type, so if you do not see the prompt right away, move around the rear and side panels and look closely at the fuel door near the wheel area.

Quick way to find it

  • Hold a filled jerry can, bottle, or canteen.
  • Walk around the Oshkosh M-ATV slowly.
  • Look for the fuel cap near the side/rear body panel.
  • Aim at the cap until the refuel action appears.

Practical note

If you are trying to refuel at a station, fill the container there first, then return to the vehicle and interact with the fuel point. The DayZ wiki also notes that fuel stations are the standard refill source on supported maps.

TL;DR: the fuel filler point is the vehicle’s fuel cap , not a separate loose part, and you usually have to be close to it with fuel in hand for the interaction to show.