Not really bad. If the oxygen sensor fails for any reason it will give readings outside the normal operating range which is set in the computer. The computer will log a fault and use some standard numbers for the O2 sensor reading. You will run slightly rich/lean, so fix it up when you can, but the car will run all day long until you can get around to it.
Clear the code (by removing the 20A engine fuse or disconnecting the battery for about 10 seconds), drive the car for a week and see if the fault comes back. If it does, replace the sensor. If it doesnt, I wouldnt worry about it. If you have the cash and dont mind paying for it, they arent too expensive and you can do it yourself.
Are we talking about your VR or VS? The VS has a heater wire, pink I think, which can cause troubles with the sensors reading. You can cut this wire to solve the problems it can cause. Search around for the exact details if you are talking about the VS.