Unfortunately, there’s not currently a way to do that inside of OmniOutliner 4; my apologies for slowing down your workflow. We do have an open request to bring tab stops to OmniOutliner 4, and if you email omnioutliner@omnigroup.com, we’d be delighted to add you to that request!
Thank you for using OmniOutliner, and for helping us make it better!