  The new Oslo Scintillator Array (OSCAR) has been commissioned at the Oslo
Cyclotron Laboratory (OCL). It consists of 30 large volume (diameter 3.5 x 8
inches) LaBr$_3$(Ce) detectors that are used for $\gamma$-ray spectroscopy. The
response functions for incident $\gamma$-rays up to 20 MeV are simulated with
$\texttt{Geant4}$. In addition, the resolution, and the total and full-energy
peak efficiencies are extracted. The results are in very good agreement with
measurements from calibration sources and experimentally obtained
mono-energetic in-beam $\gamma$-ray spectra.
