DEIMOS spectrum of SN 2018oh (gray circles) near the position of Hα. This is our first extracted spectrum where the emission line is oversubtracted. This spectrum and the spectrum resulting from the more distant background regions are shown in the inset (in black and gold, respectively). We fit a Gaussian to the emission feature and display its FWHM as vertical gold-dashed lines. The FWHM of this line is significantly smaller than of a feature expected from swept-up material (∼1000 km s−1; indicated by the vertical green-dashed lines). Data within 3 × FWHM (marked by the vertical black-dotted lines and the peach background) are outlined in red and are removed from our final spectrum. The full black circles correspond to the spectrum with 3.5 Å binning, which we use in Section 3.