The physical and chemical properties of the circumgalactic medium at z ≳ 6 have been studied successfully through the absorption in the spectra of background quasi-stellar objects1,2,3. One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers4,5,6. Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the circumgalactic medium and the intergalactic medium at z = 5–6 (refs. 7,8,9). Deep, dedicated surveys have searched for Lyα emission associated with strong C iv absorbers at z ≈ 6, but only a few Lyα-emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Lyα is a resonant line10,11,12, raising the need for complementary techniques for detecting absorbers’ host galaxies. Here we report a [C ii] 158 μm emitter detected using the Atacama Large Millimeter Array that is associated with a strong low-ionization absorber, O i, at z = 5.978. The projected impact parameter between O i and [C ii] emitter is 20.0 kpc. The measured [C ii] luminosity is 7.0 × 107 solar luminosities. Further analysis indicates that strong O i absorbers may reside in the circumgalactic medium of massive halos one to two orders of magnitude more massive than expected values8,14.