The check engine light is on on my 1998 Honda Civic DX, does it merely need to be reset using a code or is there something else. O2 sensor was replaced right before I bought the vehicle

The check engine light (yellow light) comes only when the computer control unit (called the PCM) senses a fault in the electronic controls of the engine and/or the emission system u will seldom feel the car engine run any different, there are about 200 possible faults that turn the light on, to know what has developed a problem you must have the system tested, this is called an OBD2 fault code test, Kragen and Autozone parts stores do this testing for free.