OSCAR is a MATLAB toolbox to simulate optical resonators such as Fabry–Perot cavities in presence of imperfect optics. It can handle any wavefront distortions as deviation of radius of curvature or 2D profile height of the surface of the mirrors. The package also natively supports arbitrary laser beam shapes from higher order optical modes (in Hermite or Laguerre Gauss basis) or from exotic flat beams. One of the most suitable application of OSCAR is designing and commissioning the long arm cavities of laser gravitational wave detectors.