You need to drain the radiator and do a good radiator/engine flush!
The oil and water will mix and make a gooey mess.
Can I still drive it? @Full_of_Regrets
If you drive it and heat it up so the thermostat opens, it will mix it further, so a flush would definitely be recommended, if you don't have to drive I wouldn't.
If it hasn't been started a turkey baster might get most of it out of the reservoir.
It will be a big mistake to drive it with the oil in the radiator.
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