I have never seen a vehicle where you put the oil into a reservoir tank.
Most cars have two reservoir tanks: the radiator overflow tank and the
windshield washer tank.

First, shut off the engine until you figure out what is the problem. And
don't start it until you understand the problem and there is adequate
oil in the engine and the problem fixed. In fact, I don't understand why
you started the car with a low oil level. (Now is a good time to slap
your forehead and say "DOH!".)

The oil pump will work just fine if there car is a litre low on oil. I
suspect that if you look under the car, you will discover that the oil
that you poured into the engine is, in fact, on the ground (or in the
radiator or windshield washer tank).

The light is not a low-oil light. It is a low-oil pressure light. Which
means that there is not enough oil pressure. You could have a clogged
filter or another problem, like a bad oil pump. Unfortunately, you gave
us no details like the model, year and mileage of the car that would
help us guide you better. However, if the oil level is low, chances are
that the oil pressure is low because there is too little oil to pump around.

I would bet either the drain plug or the oil filter is leaking.

Jeff

What kind of car are you working on? Are you changing oil on a vehicle
with a dry sump system? Check the owners manual for oil capacity
specifications.

Reservoir tank?  NOT for oil!  What kind and model year of automobile are
you talking about here?   And don't run the engine with the oil light on!

Most Toyotas take 5 liters to fill the oil pan, and the oil reservoir takes
anywhere from 1 to 4 liters.  Read your owner's manual for the correct
amounts.