Peugeot 308 Oxygen Sensor Error

Could be. O2 sensor issues are common. Have you checked the sensor's wiring and connectors for any damage or corrosion? It's worth a look before assuming the sensor itself is faulty. A failing O2 sensor can impact fuel trim and lead to increased consumption.

I've visually inspected the wiring and connectors, but I didn't see any obvious damage. Is there a more rigorous way to test the connection before I take it to a mechanic?

A multimeter could help you check the continuity of the wiring. However, given the symptoms and mileage, and considering you've visually inspected the connections, it's likely the O2 sensor itself is starting to fail. A workshop diagnosis is probably the next step.

I took your advice and brought it to the workshop. It turned out to be the aged oxygen sensor. The garage confirmed it with their diagnostic tools and replaced it for 95€. Everything seems to be back to normal.