  We present a detailed study of the stellar and orbital parameters of the
post-common envelope binary central star of the planetary nebula Ou~5.
Low-resolution spectra obtained during the primary eclipse -- to our knowledge
the first isolated spectra of the companion to a post-common-envelope planetary
nebula central star -- were compared to catalogue spectra, indicating that the
companion star is a late K- or early M-type dwarf. Simultaneous modelling of
multi-band photometry and time-resolved radial velocity measurements was then
used to independently determine the parameters of both stars as well as the
orbital period and inclination. The modelling indicates that the companion star
is low mass ($\sim$0.25~M$_\odot$) and has a radius significantly larger than
would be expected for its mass. Furthermore, the effective temperature and
surface gravity of nebular progenitor, as derived by the modelling, do not lie
on single-star post-AGB evolutionary tracks, instead being more consistent with
a post-RGB evolution. However, an accurate determination of the component
masses is challenging. This is principally due to the uncertainty on the locus
of the spectral lines generated by the irradiation of the companion's
atmosphere by the hot primary (used to derive companion star's radial
velocities), as well as the lack of radial velocities of the primary.
