Technically speaking, the Object Windows Library (OWL) is a Borland C++ object-oriented framework originally designed for WinAPI. It was used in Turbo Pascal for Windows, Borland Pascal and their Borland C++ package. It was a competitor to the Microsoft Foundation Class Library (MFC).
According to Microsoft, OWL had incomplete support by the Borland C++ IDE and wasn't always upwardly compatible from release to release. It was eventually deprecated in favor of the Visual Component Library (VCL), which is written in Object Pascal and included in Delphi and C++ Builder Studios.
An open source Internet community has released a series of improvements and patches to the original OWL; the project name is OWLNext.