Designer Chris Micallef discusses the Oops command in AutoCAD and how it differs from the Undo command.

"Does anyone remember the Oops command? Just type it on the Command line, and it will undelete the last selection that was erased using the Delete command. I don't use the command very often, but occasionally I have erased something and continued working only to realize that the deleted geometry is still required or was erased by mistake. Oops will perform an undelete, avoiding multiple Undos. Remember, unlike Undo, Oops can only be performed once. In other words, only the last selection set that was deleted can be resurrected. Any previous deletions can only be restored by Undo."
 
Notes from Cadalyst Tip Patrol: Oops is an easy command to forget for some reason. Instead of using it, we tend to Undo everything until our deleted objects have returned. But that can eliminate work that you still need. You can also use Oops after Block or WBlock, because these commands can erase the selected objects after creating a block. However, you cannot use Oops to restore parameters, actions, or grips in the Block Editor. You cannot use Oops to restore objects on a layer that has been removed with the Purge command.