I use both applications concurrently, and I'm not seeing OmniFocus do this on my machine; next time it happens, use the Activity Monitor application in your utilities folder to take a Sample of OmniFocus - that way we can at least see what it's doing with all that CPU.
To do this, you'll want to do the following:
1) Go to your Applications folder --> Utilities folder--> and launch "Activity Monitor".
2) A window will open that shows all the applications running on your machine. In the list, select our application.
3) Click on the "Inspect" button on the toolbar
4) Another window will pop-up. Click the "Sample" button on the lower left corner, switch to OW, and type into the text box.
5) When Activity Monitor is done taking the sample, click the "Save..." button on the top right and email the document it creates to firstname.lastname@example.org
. We'll have a look at it and see if we can figure out what's going on here.