I found the solution. Just needed to filter the data before going to the Reports Window.
I have been using OmniPlan for a while and I could not find a function to filter the data of the reports in the Reports Window.
Actually I need to report the Resources of the projects once per month like who did what tasks and how many working hours they spent in a month.
That would be great if I am able to filter the tasks within a date range (as MS Project is supporting) so my report will be more straightforward.
Can you please give me an advice?