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We present a sample of four emission-line galaxies at z = 6.11–6.35 that were serendipitously discovered using the commissioning data for the James Webb Space Telescope (JWST)/NIRCam wide-field slitless spectroscopy mode. One of them (at z = 6.11) has been reported previously, while the others are new discoveries. These sources are selected by the secure detections of both [O iii ] λ 5007 and H α lines with other fainter lines, which were tentatively detected in some cases (e.g., [O ii ] λ 3727, [O iii ] λ 4959). In the [O iii ]/H β –[N ii ]/H α Baldwin–Phillips–Terlevich diagram, these galaxies occupy the same parameter space as that of z ∼ 2 star-forming galaxies, indicating that they have been enriched rapidly to subsolar metallicities (∼0.4 Z _⊙ ), similar to galaxies with comparable stellar masses at much lower redshifts. The detection of strong H α lines suggests a higher ionizing photon production efficiency within galaxies in the early universe. We find brightening of the [O iii ] λ 5007 line-luminosity function (LF) from z = 3 to 6, and weak or no redshift evolution of the H α line LF from z = 2 to 6. Both LFs are underpredicted at z ∼ 6 by a factor of ∼10 in certain cosmological simulations. This further indicates a global Ly α photon escape fraction of 7%–10% at z ∼ 6, which is slightly lower than previous estimates through the comparison of the UV-derived star formation rate density and Ly α luminosity density. Our sample recovers ${66}_{-44}^{+128}$ % of z = 6.0–6.6 galaxies in the survey volume with stellar masses greater than 5 × 10 ^8 M _⊙ , suggesting the ubiquity of strong H α and [O iii ] line emitters in the Epoch of Reionization, which will be further uncovered in the era of JWST.