Is any NURBS support in OpenCascade?

Yes!
Geom_BSplineCurve and Geom_BSplineSurface are NURBS curves rsp. surfaces. You can have rational (with weights!=1.0) and nonrational BSplines. They can be open or periodic (in NURBS literature often called unclamped and clamped)
best regards
Commenter-1

Can NURBS surfaces also have a "UV-trimcurve"?
I'm new to OpenCASCAE and had a brief look at the API.
I didn't find any method to define a trimcurve. It seems the surface can only be bounded by a UV parameter range.

Yes you do, but it is not specific for nurbs, you use a wire to do the trimming and that works on any parametric surface, so you need to use the topology classes to trim with curves, wires can be used to trim the outside of a face and also to create holes depending on orientation.