If you really must be in quarter scale, you can put the months in the quarter header (choose "Beginning of Quarter" and "End of Quarter") but they won't necessarily line up with the relevant months.
If you're using the 1.5 betas, a better approach might be to switch to month scale.
1) Use the magnifying glass to select "month" scale.
2) Then click and drag the gantt header to make it as narrow as the quarter scale would have been.
3) Open Preferences from the OmniPlan menu
4) Click on the "Display" icon to open that tab.
5) Change the "Customize headers for" drop-down to "Months"
6) Click the plus button next to "Summary header format"
7) The summary header should now show quarters and the date header should show months (unless you've customized them previously)
8) Customize the tokens as you choose
9) Close the preferences pane
If that doesn't get you close to what you want, please try explaining more about what you want to see.