  We present the design details of oil-coupled lens groups used in the KOSMOS
spectrograph camera. The oil-coupled groups use silicone rubber O-rings in a
unique way to accurately center lens elements with high radial and axial
stiffness while also allowing easy assembly. The O-rings robustly seal the oil
within the lens gaps to prevent oil migration. The design of an expansion
diaphragm to compensate for differential expansion due to temperature changes
is described. The issues of lens assembly, lens gap shimming, oil filling and
draining, bubble mitigation, material compatibility, mechanical inspection, and
optical testing are discussed.
