I have a Mitsubishi Lancer 1998 1.3 75HP car and recently it started getting very low mileage. When I went to a technical examination of the car where they connected it to the exhaust gases tester, they noticed that the O2 level was very high.

Spark plugs have been changed recently. It started happening a few months ago, when someone else started to drive the car (I don't know if that's relevant). Have you any idea what could be the problem?

All other exhaust gases were perfectly OK... HC, CO, CO2... only O2 was problematic (very very high). How could I test if the oxygen sensor is bad?

Thoughts:
1. Thoroughly warm up engine, attach a scanner, and see if it's throwing a HO2S code.
2. Determine how many oxygen sensors the car has. Some cars have additional sensors located on the catalytic converter. Remove oxygen sensor(s) pigtail connector and carefully dress the contacts with jeweler files. Don't use contact cleaner, brake cleaner, WD-40, or anything on the contacts or the ECM will throw a code forever. It's very unusual for a low mileage car to have an oxygen sensor problem. High oxygen level readings indicate a lean burn situation; i.e. high oxygen/fuel ratio. Therefore, I would next:
3. Add Regane or Techron to the gas and clean the fuel injectors and throttle body. This may help restore the proper oxygen/fuel ratios.
4. Oxygen sensor tests are engine specific, even within the same make of car. Most tests proceed along these lines. Reset ECM, warm up engine, and see if a code is thrown. If no code is thrown, it's an intermittent failure, where you check for poor connections or loose wires at the connector or ECM. If a code is thrown, disconnect the pigtail connector from the HO2S and measure resistance between terminals C and D on the HO2S (e.g. 10-40 ohms). If resistance is outside these parameters, replace. If resistance readings are OK, check for continuity to body ground on terminals C and D of the HO2S. Other specific tests go on from there. You would need to get the specific tests for your HO2S. I couldn't find any information on a 1998 Mitsubishi Lancer--it must not have been sold in the U.S.
4 Pin Connector:
AB
CD
Some sensors are 8 pin, rather than 4 pin:
ABCD
EFGH

Tests proceed along and ultimately pass judgment on the ECM.
5. Have your engine thoroughly warmed up (out of "open loop" mode), before you have a technical examination.