  We describe the design and in--flight performance of the cryostat and the
self-contained $^{3}$He refrigerator for the OLIMPO balloon--borne experiment,
a spectrophotometer to measure the Sunyaev-Zel'dovich effect in clusters of
galaxies.
  The $^{3}$He refrigerator provides the 0.3 K operation temperature for the
four arrays of kinetic inductance detectors working in 4 bands centered at 150,
250, 350 and 460 GHz. The cryostat provides the 1.65 K base temperature for the
$^{3}$He refrigerator, and cools down the reimaging optics and the filters
chain at about 2 K.
  The integrated system was designed for a hold time of about 15 days, to
achieve the sensitivity required by the planned OLIMPO observations, and
successfully operated during the first long-duration stratospheric flight of
OLIMPO in July 2018.
  The cryostat features two tanks, one for liquid nitrogen and the other one
for liquid helium. The long hold time has been achieved by means of custom
stiff G10 fiberglass tubes support, which ensures low thermal conductivity and
remarkable structural stiffness; multi--layer superinsulation, and a vapour
cooled shield, all reducing the heat load on the liquid helium tank.
  The system was tested in the lab, with more than 15 days of unmanned
operations, and then in the long duration balloon flight in the stratosphere.
In both cases, the detector temperature was below 300 mK, with thermal
stability better than $\pm$ 0.5 mK.
  The system also operated successfully in the long duration stratospheric
balloon flight.
