An OCX is an an ActiveX Control derived from the OLE paradigm. It is based on the COM (Component Object Model) and DCOM (Distributed Component Object Model).
An ocx is an an ActiveX Control derived from the OLE (Object Linking and Embedding) paradigm. It is obviously based on the COM (Component Object Model) and DCOM (Distributed Component Object Model) specification.
These controls are not standalone applications, but modules usually available as DLLs that expose functionality through the property, method, and event interfaces.