The Legacy Survey of Space and Time (LSST) aims to measure the fluxes of astronomical sources with sub-percent precision and their morphologies with percent-level precision. Uncorrected interference fringing in the LSST detectors by night sky NIR emissions lines could undermine these goals. Although the interference patterns of LSST detectors are well characterized as functions of wavelength, removing interference fringes from an image is possible only if the intensity of the fringing emission is known. And because the intensities of sky emission lines change continuously and unpredictably, properly correcting for their effects on an LSST image will require knowledge of the line intensities at the time of the observation. Here, we introduce the Overall Sky Emission Line Observing and Tracking Spectrograph (OSELOTS), a self-contained optical to NIR spectrograph designed to solve this problem. In regular operation, OSELOTS produces a calibrated spectrum of the diffuse night every ten minutes, providing near-live measurements of all emission lines that are at least 20% brighter than the night sky continuum. By integrating these timestamped OSELOTS spectra into the LSST data reduction pipeline, the LSST analysis team will deliver data products that are free of interference fringes sourced by sky emission lines. We have already used OSELOTS to track the varying intensities of sky emission lines during fifty nights of observing in Flagstaff, AZ. We will deploy OSELOTS to Chile later this year, where it will be integrated into the ongoing Rubin Observatory commissioning process.