  The Compton Spectrometer and Imager (COSI) is a balloon-borne soft
$\gamma$-ray telescope (0.2-5 MeV) designed to study astrophysical sources.
COSI employs a compact Compton telescope design and is comprised of twelve
high-purity germanium semiconductor detectors. Tracking the locations and
energies of $\gamma$-ray scatters within the detectors permits high-resolution
spectroscopy, direct imaging over a wide field-of-view, polarization studies,
and effective suppression of background events. Critical to the precise
determination of each interaction's energy, position, and the subsequent event
reconstruction are several calibrations conducted in the field before launch.
Additionally, benchmarking the instrument's higher-level performance through
studies of its angular resolution, effective area, and polarization sensitivity
quantifies COSI's scientific capabilities. In May 2016, COSI became the first
science payload to be launched on NASA's superpressure balloon and was slated
for launch again in April 2020. Though the 2020 launch was canceled due to the
COVID-19 pandemic, the COSI team took calibration measurements prior to
cancellation. In this paper we provide a detailed overview of COSI
instrumentation, describe the calibration methods, and compare the calibration
and benchmarking results of the 2016 and 2020 balloon campaigns. These
procedures will be integral to the calibration and benchmarking of the NASA
Small Explorer satellite version of COSI scheduled to launch in 2025.
