  We have investigated solid oxygen as a converter material for the production
of ultra-cold neutrons. In a first series of experiments the crystal
preparation was examined. An optically semi-transparent solid $\alpha$-oxygen
crystal has been prepared. In a second series of experiments such a crystal
prepared indentically as in the first series of experiments has been exposed to
the cold neutron flux of the MEPHISTO beam line of the FRM II. Ultra-cold
neutrons produced inside the oxygen crystal have been extracted and the count
rates have been measured at different converter temperatures. The results of
these measurements give a clear signal of the superthermal UCN production
mechanism in $\alpha$-oxygen. The mean free loss length of UCN inside the
crystal at a temperature of 5\,K was determined to be in the order of
$20\,\mathrm{cm}$.
