What will still work

As long as the C++ application on OpenServer or UnixWare 2.x is a completely bound executable in C++ terms, it can run on UnixWare 7. This means the application is an a.out executable, with any dynamic libraries containing only C code. Thus:

   uw-machine:  CC -o hello hello.C
   uw-machine:  rcp hello gemini-machine:
   

   gemini-machine: ./hello
   Hello, UnixWare 7

This also assumes that application is not doing anything at the C level that would prevent it from running successfully on UnixWare 7, e.g. all the usual C-level compatibility constraints still apply.