VisualAge C++ is the best development tool to use in OS/2. It comes with full featured IDE, Debugger, fast Compiler and Linker, Performance Analyser, Visual Builder, Data Access Builder and OpenClass libraries. Version 4 also comes with Warp 4 Toolkit which contains an Assembler, Help File compiler and Resource File compiler as well as a bunch of libraries, include files, API documentation and samples.
Very discounted academic versions are also available, check with software vendors.
Requires various files from FTP site
EMX is a free development package composed of GNU utilities such as GCC and GAS who uses EMX libraries made by our beloved Eberhard Mattes. EMX is best used to port Unix programs to OS/2 as the functions are almost all compatible and implemented the same way, and the compiler used, GCC, is the same on Unix platforms, or at least compatible. EMX can also be used to start a new project from scratch in OS/2 or Win32, but it is not well suited for this purpose as threads and DLLs sometimes behave strangely using EMX, and it does not come with a full featured debugger. But nonetheless, for its price, EMX is a must have.