Mouseovers and rollovers provide a visual cue that an object is interactive and are core requirements in web design and multimedia authoring to ensure good user experience (UX). So with Opus they're built in.
Any object can have a set of appearances designed to display when the mouse moves over, when a button is down or when the object is clicked on/disabled/being dragged. No programming is required and you can preview how it will look immediately.
Set a flare, stretch the object slightly or change the border - it's all as easy as designing the properties of the object in the first place and Opus takes care of the scripting automatically.
If you do want to design special events for rollovers there are dedicated MouseOver and MouseExit triggers and even a special Rollover version of multiframes - ideal for displaying tooltips or other hints.
Opus Pro and Opus Creator Reference - MouseOvers and Other Appearances