mercury mountaineer o/d light flashing?

This OD light flashing means a fault has has been sensed by the transmission control module in the electronic transmission controls, to see what this fault might be have the transmission tested for electronic fault codes that are stored when this OD light flashes, many times this code will be for a transmission slipping issue, u will not fell this most times, if the transmission has more than 60,000 miles on it have the transmission fluid and filter changed and erase the codes, they may not come back.

service engine soon light comes On in Ford 2002 Explorer?

the service engine soon light is not for the oil or anything like it, it is warning u that the electronic engine control system has a fault present, there are about 200 poss reasons so you will need to get the system teasted, Kragen and Autozone both do this test for free, just ask for a OBD 2 fault code test, don't listen to the suggestions of what is wrong, they are not trained in this and often you end up buying parts that don't fix the problem, seek a Ford dealer out or other trained electronic engine control tech for assistance in fixing the problem.