1990 pontiac grand am starts ok put it in gear

Does the service engine come on? there are a few thins that could cause this, i'm guessing it is an automatic, the oxygen sensor can cause an aray of problems, the tps, more than likely the transmission converter clutch solenoid valve or tps. the tps is an easy repair, the tcc is also fairly easy, on the side of your tranny there is another pan, you take that off to gain access to the tcc. you can test to see if it is the tcc by simply unplugging it. if unplugging it does not work check your tps it will definately cause this.