To make Divi (and themes based on Divi, like Extra) work with OMGF or OMGF Pro make sure Divi > Theme/Plugin Options > General > General > Use Google Fonts is enabled. OMGF (Pro) replaces Google Fonts on-the-fly. Which means it needs to know where Google Fonts are located in order to replace them.
Divi's big update introduced many changes; how it handles Google Fonts is one of them. You'll have to disable the improve Google Fonts loading setting under Divi > Plugin Options > Performance to make sure OMGF (Pro) is able to detect the fonts.
Sidenote: there have been reports that the limit Google Fonts support for Legacy Browsers option (available in older version of Divi) also needs to be disabled, so I've added it here for full disclosure. In most cases, however, this shouldn't make any difference.
The Extra theme also offers the same features as Divi, the options to disable are located under Extra > General > Performance.