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In the calendar view, choose "Normal Work Week for Project" from the drop-down in the lower left corner. Now edit the green rectangles to match your working hours. You can resize the existing ones, draw new ones, or shift-drag to erase portions of the rectangles.

You may also want to use the Project formatting inspector to adjust the 'days per week' setting. If you're working 3 8-hour days per week, this should be 24 instead of 40.

These features are also covered in Step 5 of the Tutorial in the OmniPlan help.