Download and install the free WordService from Devon Technologies (makers of DevonThink, DevonNote, DevonAgent, etc.) at
http://www.devon-technologies.com/pr.../services.html.
Once WordService is installed, all you need do is highlight the text you wish to capitalize in OmniOutliner, then from the OmniOutliner menu select Services > Convert > All Caps. WordService includes a host of other useful text manipulation services (including a word count feature neatly hidden under a service titled Statistics).
I have tested it with OmniOutliner Pro and it indeed works as advertised.