The parish church of San Marco stands at the heart of Onzo's medieval village centre, its Romanesque-influenced campanile visible from the surrounding hillsides. Like most churches in the Ligurian hinterland, it has been altered over centuries — the current building incorporates medieval fabric with later Baroque additions to the interior — but the setting against the old stone houses of the village is attractive.
The church is generally open in the mornings. Inside, there are some modest decorative details and a quiet, cool interior that provides welcome relief in summer. The small piazza in front of it is the social hub of the village, where elderly residents gather in the late afternoons.