The Compact Muon Solenoid (CMS) experiment plans to replace its strip tracker system with a completely new Outer Tracker system to cope with the higher luminosity, compared to Run 2 operation, provided by the HL-LHC. This CMS Phase-2 Outer Tracker will be build up from two types of modules both consisting out of two parallel silicon sensors separated by a few millimetres. To read out the two types of modules four Outer Tracker specific custom chips are required. This proceeding introduces the module concept, goes into more detail on the data path, discusses the test system (OT-mu DTC) designed for testing prototypes based on these ASICs and gives examples of test results obtained with this test system.