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 objects1-3. One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers4-6. 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. 7-9). 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 line10-12, 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.