Different menu — not the one reached by the button marked ‘menu’
To the left of the hazard light switch are three buttons marked with arrows for ‘up’, ‘down’ and ‘set/return’.
With the ignition on but engine off, drivers door closed and (crucially) the headlights off*, press ‘set’ to show the menu, which will appear above the fuel gauge. The up and down buttons let you move through the options. Use the ‘set’ button (also marked with the arrow for ‘return’) to confirm settings, which should include the reset you’re looking for.
By the way, that warning doesn’t necessarily mean you need to add any oil — there’s no electronic dipstick. It simply comes on every few thousand miles as a reminder (even if you have checked it) because oil level is so critical in the TwinAir. Check the level carefully before deciding if you need to add any. Filling it does not clear the warning. Other models with the 1.2 or diesel don’t have this warning.
(* When the headlights are on, the up and down buttons are instead used to set the beam level according to the weight in the car, shown by the 0, or 1-3 next to the headlight symbol under the fuel gauge when the lights are on)