Selecting files with the mouse or keyboard, in all view modes except Power mode, works similarly to Explorer.
Note that some mouse drivers map the middle mouse button to another function by default - if you find middle-click isn't working as expected, see the FAQ for some hints on how to fix it. You can also configure Opus to treat a single middle-click as a double middle-click, which lets you obtain behavior similar to a web browser (e.g. single middle-click to open a folder in a new tab). See the File Displays / Mouse Preferences page for details on that.
In power mode, mouse and keyboard selection works slightly differently by default (although the behaviour of the mouse can be modified quite a lot from the File Display Modes / Power Mode Preferences page).
The default behaviour of power mode is for file selections to be persistent (that is, files do not deselect by themselves - you have to deselect them manually). In other file display modes you can simply click on an empty area to deselect all files, but in power mode: