The -width option on tk_optionMenu results in a widget that is far too narrow to display that many characters on MacOS X. If -indicatoron is 1 the result is even worse (even less of the text is displayed). For example:
tk_optionMenu .om omVar DC1 DC2 DC3
.om configure -width 3
this displays as expected on Linux, but the widget is far too narrow on MacOS X.