Check engine light came on, diagnostic revealed oxygen sensor, replaced all oxygen sensors, drove the car for 50 kms, stopped and restarted the car and after a few minutes, the light came on again. What is the real problem?

The beginning of yourproblem is misdiagnosis, you probably had it code scanned,and got an 02 code, if you had it on a real time scanner you would get readings from the scanner to assist in diagnosis, story i tell my assistants, the floor is wet, first guy mops it up and leaves, second guy noticed water dripping on the floor, so he mops it and places a bucket to catch the drips. third gut sees the wet floor, notices the drip , looks up an notices the roof is leaking, so he fixes the leak and mops the floor, same idea with cars, you get the symptom but you have to find the cause to repair right, so find a tech with a real time scanner and take the time to do it right