IPMD, a PM-based debugger, ships with C-Set++. It is capable of source- and assembly-level debugging multithreaded 16 bit and 32 bit OS/2 applications
emx/gcc comes with gdb 4.11, the GNU debugger.
Borland C++/2 comes with a PM-based debugger called Turbo Debugger GX which has the same basic functionality as IPMD.
There are also several commercial debuggers on the market. WATCOM C and FORTRAN come with WVIDEO, a full-screen source or assembly level debugger that handles multithreaded 16 and 32 bit OS/2 programs. Multiscope (no longer available for 32-bit OS/2) and Periscope are others. CodeView is still applicable for 16-bit OS/2 apps, but it is not recommended for 32-bit apps.