Opus allows you to centre objects horizontally between the left and right edge of the page, vertically between the top and bottom of the page or to the exact middle of the page. Opus provides a shortcut via the Centre Objects option on the Arrange menu.
Hold down the Shift key and click on all of the objects you want to centre. A bounding box will surround each object.
Click on the Arrange menu and point to Centre Objects, then click on the Across, Down or Both option. The objects will be centred in the direction you specified.