@payam, look for the Low Temperature Radiant options (Electric, Variable Flow, Constant Flow) in the OpenStudio App (zone equipment, under the "Thermal Zones" tab). You'll likely also need one or more Internal Source Constructions (under the "Constructions" tab). Unless you're after the Electric option, you'll of course need to set up a plant loop as well.

The easiest is to follow the logic/sequence described in any of the lowtemprad examples here, e.g. lowtemprad_electric.rb. You can also open the corresponding .osm file and run a trial simulation (to familiarize yourself with it all). Of course, this section and this section of the IO Reference are essential reading.