Turns out we accidentally broke the Quick Look generator in beta 4 when it's used from app, which all of you should be doing. This unfortunately slipped passed us due to the fun involved in testing the Quick Look generator when there's a million different versions of the app available. So just a day later, beta 5 is now available with working Quick Look support! There's also a cool new feature to it that was actually in beta 4. If you mouse over the bottom center of the Quick Look preview, two buttons will appear for expand and collapse all. Hurray for software updater!
Please see the release notes for full details. Head on over to the download page when you're ready, or use the built-in updater if you're using a 3.9 beta (OmniOutliner menu -> Check for Updates). If you have any feedback on this release, please let us know by using the Send Feedback option under the help menu or send an email to firstname.lastname@example.org. Thanks!