Get an oil change carried out - in future, at least every 10,000 miles, or every year, WHICHEVER COMES FIRST, get an oil change done.

Saving money on servicing is the one sure way to end up costing you more money in the long run

Yes, the OP should have done an oil and filter change rather than simply top up the oil, but he asks as to how he can extinguish the warning light. After all, the little yellow 'pump' symbol will light up when you're low on fuel, but filling up will put out the light. You wouldn't want to be paying a dealer to reset the 'pump' light after every fill up. Why then must we pay the dealer to reset the 'oil service' light after we've changed the oil, or, for instance, the light which comes in when the Eolys Fluid level is low and we refill the Eolys tank? We need someone to manufacture a device which plugs into the car and resets these warnings! Or perhaps such a device is available but I haven't yet seen one?

You have it spot on sir! The first hit on a Google search is a Toyota forum, where the contributors say that the method of turning off the warning light is in the 'owners' handbook'! No, I've never read my owners' handbook either!

Seems you need to have more than two hands to operate the ignition key at the same time as the odometer button, but ensuring that the button is set to 'odometer' and not 'trip'.