By default, Open Corresponding File (Alt+O) searches for related files in locations associated with the current project, as well as .., ./inc, and ./include. For example, one can navigate among the following:
/foo/bar.cpp
/foo/bar.h
/foo/baz.cpp
/baz.h
You can omit .., ./inc, and ./include from searches, especially if their sizes impede the performance of Alt+O, by modifying the registry.
HKCU\Software\Whole Tomato\AggressiveFileMatch = 00
Set the value to 01 to restore default behavior, i.e., search also in .., ./inc, and ./include.