You’ll have to create a new one since OrderedDict is sorted by insertion order.
In your case the code would look like this:
foo = OrderedDict(sorted(foo.items(), key=lambda x: x[1]['depth']))
See http://docs.python.org/dev/library/collections.html#ordereddict-examples-and-recipes for more examples.
Note for Python 2 you will need to use .iteritems() instead of .items().