  Free-electron lasers (FELs) have been designed to operate over virtually the
entire electromagnetic spectrum from microwaves through x-rays and in a variety
of configurations including amplifiers and oscillators. Oscillators can operate
in both the low and high gain regime and are typically used to improve the
spatial and temporal coherence of the light generated. We will discuss various
FEL oscillators ranging from systems with high-quality resonators combined with
low-gain undulators to systems with a low-quality resonator combined with a
high-gain undulator line. The FEL gain code MINERVA and wavefront propagation
code OPC are used to model the FEL interaction within the undulator and the
propagation in the remainder of the oscillator, respectively. We will not only
include experimental data for the various systems for comparison when
available, but also present for selected cases how the two codes can be used to
study the effect of mirror aberrations and thermal mirror deformation on FEL
performance.
