[jb92563]

Its not the end of the world, dont run the engine yet though.

It is a coolant overflow tank so if you have not run the engine much chances are it may not have entered the coolant system proper, especially since oil floats.

If the oil has circulated through the engine then flushing ALL your coolant out
and getting the oil out of the coolant system is necessary.

Start by removing the coolant tank hose from the engine and flush the coolant tank with sudsy dish soap water until the oil is gone.

If there is no oil in the coolant then you are good to reconnect and top up the coolant tank.

If there is oil in the coolant then a complete flush of coolant will start to process of riding all the oil from your system.

I doubt a trace of oil will do any harm since the boiling point of oil is higher than the boiling point of the coolant, but if it were me I would try to get ALL the oil out by trying to use some degreaser to clear all the oil out.

I'd ask an indie mechanic how they deal with that.