When i turn off my car it hesitate to completely cut off even though it cranks smoothly. What could it be

Hello, first why did you replace the oxygen sensor? You're saying when you shut the vehicle off, it keeps running for a couple seconds before it finally stops. I would be focused on the ignition system, as it's not completely shutting off the vehicle as it should. Some testing around that area and the engine performance also seems to be in order.

IF someone has hard wired the radiator cooling fan to turn on any time key is on that will cause this concern. Fan motor becomes a voltage generator while blades are rotating - when key is turned off this voltage is enough to back feed the ignition until fan blades slow to a particular rpm. Just one possible scenario.