What kind of oil? The brake system is totally independant of any other system. The only way stuff gets in is via the cap in the reservoir for the master brake cylinder.
If the brake fluid has never been changed over the life of the car, perhaps the brake fluid separated due to age. Perhaps someone poured in motor oil, or power steering fluid into the brake reservoir when they “topped off” the fluids.
If the fluid in the system didn’t deteriorate due to age, then someone added the wrong fluid. The timing of the oil change coincides neatly with brakes failing 3 days later.
Therefore my “Sherlock Holmes” answer is the oil change person(s) at the shop messed this one up.