If it’s a dynamic panel or a group, you can choose “hide from view” in the outline view (in addition to setting the “hidden” property). It’s at the far right of the item’s entry in the outline view. The trouble with this is that if you have a lot of stuff on your page, it’s hard to find the “hide from view” widgets.
An alternative is you can move the hidden object offscreen to the right (since it’s hidden, it won’t affect the page size) and then move it to where you want it to be in its OnLoad interaction.
In a push/pull situation, you can start with the hidden widget showing (and with the widgets below in the spots they’re supposed to be when that widget is showing) and OnPageLoad hide the object, pulling widgets below. (This will happen before the page is displayed.)