OrderedMultiDict is not used by Werkzeug. It has a complex implementation, and is documented to be slow. dict maintains key insertion order in all supported versions of Python. MultiDict already maintains the insertion order of values within a key. OrderedMultiDict maintains the insertion order of each item, so [(a, 1), (b, 2), (a, 3)] comes out the same instead of [(a, 1), (a, 3), (b, 2)]. It's not clear there's a use case for this, searching sourcegraph shows some use of OrderedMultiDict but it's not clear if the use is actually required vs using MultiDict. Other libraries such as boltons also provide implementations. Mark this as deprecated and see if anything comes up.