Oxygen Sensor and (OVP Relay?) for 1990 200E
 

I have all the symptoms of a needed Oxygen Sensor replacement, is it simple to replace once you have access to the car underside? Do I really need to have the computer reset once the replacement is done? I also been trying to find out through some thread about OVP Relay, what does OVP stands for, because my car also carry the symptoms for the OVP replacement i.e. ; hard start when engine is warm.
Thanks in advance and will appreciate any response

Ed
1990 300E 116,000 miles:confused: