Go to your organisation page (click on the icon in the top-right corner of your organisation page, select the drop-down menu)
Go to the "Settings" tab and then press the "Customize" tab
Here you can edit the functionalities of your organisation page:
Members can create opportunities
When this functionality is active (slide to turn coloured), members of your organisation can create opportunities and events. When this functionality is inactive, only moderators and admins of your organisation can do so.