In a comment on my Five More Simple Tips for AutoCAD post, Jeremy had some questions about the Oops command. I thought I would explain the command in more detail in another post so more people will see it.
The first thing to understand is that Oops varies from Undo/U in that it only reverses those commands that erase objects. The Erase command is obvious, and the same applies to pre-selecting objects and hitting the Delete key, but the Block and Wblock commands can also erase the objects that go to make up the block. In such cases Oops can be used to restore those objects without undoing the block creation step.
Another difference is that unlike Undo, you can only go back one step with Oops. If you erase one set of objects and later erase another set, issuing two Oops commands will not restore the first set of objects. Where Oops has the advantage is that it can unerase something that you erased 100 commands ago, without undoing the intervening 99 commands.
What happens if you use Oops when there is nothing to unerase? Nothing. Oops simply returns to the Command prompt. Also, there is no short form of the Oops command in standard AutoCAD. O invokes Offset, OO does nothing and OP invokes Options.
Finally, the U and Undo commands interfere with Oops. I suspect this is what Jeremy was seeing when the command wasn’t working for him. To see what I mean, select and right-click > Copy the following commands. In an empty drawing, right-click > Paste them into the Command prompt area:
_.Zoom _Win 0,0 10,5
_.Line 1,0 1,2
_.Line 2,0 2,2
_.Line 3,0 3,2
_.Line 4,0 4,2
_.Erase 1,1 2,1
_.Line 5,0 5,2
_.Line 6,0 6,2
_.Move 5,1 6,1 5,3 7,3
This command sequence zooms to a suitable area, draws four lines, erases the first two, draws another two and then moves them to the right. If you use Oops immediately after that, the first two lines will return. However, if instead of doing that you first undo the Move command and then use Oops, the lines will not return.