Setting work hours?
 

I am working in a project for three days in the week. Is it possible to view only the days I am working in this project.

For example: I work monday, tuesday en friday. When I plan a task for 24 hours starting on monday I want the it fills in tuesday and friday automatically.

Hope this is working because it is to much work to plan tasks per day.

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.