  OSIRIS is a near-infrared (1.0--2.4 $\mu$m) integral field spectrograph
operating behind the adaptive optics system at Keck Observatory, and is one of
the first lenslet-based integral field spectrographs. Since its commissioning
in 2005, it has been a productive instrument, producing nearly half the laser
guide star adaptive optics (LGS AO) papers on Keck. The complexity of its raw
data format necessitated a custom data reduction pipeline (DRP) delivered with
the instrument in order to iteratively assign flux in overlapping spectra to
the proper spatial and spectral locations in a data cube. Other than bug fixes
and updates required for hardware upgrades, the bulk of the DRP has not been
updated since initial instrument commissioning. We report on the first major
comprehensive characterization of the DRP using on-sky and calibration data. We
also detail improvements to the DRP including characterization of the flux
assignment algorithm; exploration of spatial rippling in the reduced data
cubes; and improvements to several calibration files, including the
rectification matrix, the bad pixel mask, and the wavelength solution. We
present lessons learned from over a decade of OSIRIS data reduction that are
relevant to the next generation of integral field spectrograph hardware and
data reduction software design.
