Hi Mike,
In our CMakeLists.txt file, there is a "profile" option that adds flags necessary for doing profiling (if using gcc). When you're running cmake, add -Dprofile=on to make sure you get these flags. I believe gprof doesn't work well for us currently because most of OpenMC is built as a shared library. If you changed it to build libopenmc as a static library, you might be able to get gprof to work. Generally I use the Linux perf profiler for OpenMC, so the workflow looks something like:
cmake -Dprofile=on ..
make
cd <directory_with_inputs>
perf record -g openmc
perf report
I know myself and others have also used Intel VTune with success, if that's an option for you.
Best regards,
Paul