97 malibu oxygen sensor codes.

my 97 malibu (95K)is throwing an oxygen sensor code along with 4 "dealer specific codes"

The drivablity is fine, runs like it always has, except the check engine light is on.

Would it be safe to assume it may be the Oxygen sensor?

Do I have to go underneath the car to access the sensor so I can test it.

I used one of those cheap diagnostic computers. If I just take it to a regular garage that has a good one ie. snap on, will that know the dealer codes?

Thanks for the help!