There are two features in Opus that help you keep track of changes you have made to files and folders.
The Undo command, as you would expect, can usually be used to undo the most recent action. Most actions can be undone - for example, if you rename a group of files incorrectly with a wildcard, you can undo the rename fairly easily. File deletes, however, can not be undone unless the delete was done to the recycle bin. You can find the Undo command in the Edit menu in a Lister (or press Ctrl+Z).
Opus actually maintains a history of undoable functions. From the Undo List sub-menu, you can see the ten most recent actions, and undo any of them by selecting it from the list. Selecting the More... command from that sub-menu displays the Utility Panel, showing the Undo Log page. This page displays a full list of undoable actions, and you can undo one, many or all actions from this page at the click of a button.
While the undo log only shows the most recent undoable actions in the current session, the file log maintains a persistent log of all file operations (undoable or not). You can access the File Log from the Utility Panel, or select the command directly from the Help menu.
By default the file log records the last 1000 file operations. You can see
what sort of action occurred, the file involved, when it happened, and in the
case of a copy or move, the destination folder.
The file log can be extremely useful at times - if you've ever found that a file you knew was there has mysteriously disappeared, you can often find out what happened to it through the log.
The 
button at the top of the log lets you
export the log as a text file - useful if it's quite large and you want to
search it in a text editor. The 
button is used
to clear the log. You can configure the maximum size of the log, and control
which operations are recorded, from the File Operations / Logging page in
Preferences.