There seem to be two ways of doing this.
In case Pauli's instructions disappear, here's just the text from his explanatory page:
Let's say we have map versions A and B. They are almost identical with only some differing elements, for example some incremental modifications in the vegetation or so. No symbols or colors have been modified. This is how to proceed to reveal those differences:
Now, in front of you, you have only those objects that are in the map B but are not in the map A. Before saving the result under a new filename (File - Save As), you might do some tidying: remove all background maps, delete unused symbols and colors, and optimize the map to not unnecessarily save all those deleted objects that will always remain hidden in OCAD's object storage until you optimize them away.