What is there

• defaultdict(factory[, mapping]): a missing key on a subscript READ calls the factory, stores and answers the value. get(), in, pop() and setdefault() do not call it, as in CPython (only __getitem__ consults __missing__). Any callable value works -- int, list, set, str, dict, a def, a lambda, a class -- through a new zero-argument hook (PyCall0Hook = pyeval's pyvar_callv0). defaultdict() / defaultdict(None) raise KeyError like a dict. copy() keeps the factory; dict(d) does not. repr is CPython's defaultdict(<class 'list'>, {...}); type name defaultdict; isinstance(d, dict) is True.
• OrderedDict([src]): a dict already keeps insertion order, so the mode changes the type name and repr (OrderedDict({...}), 3.12+ format) and adds move_to_end(key, last=True) and popitem(last=True), which are dict methods and harmless on any dict. move_to_end(last=False) is O(n), where CPython's linked list is O(1); an LRU moves to the END, the cheap direction.
• Equality with a plain dict is order-insensitive, as CPython's OrderedDict == dict is. OrderedDict-vs-OrderedDict order-SENSITIVE equality is NOT implemented (it compares as dicts).

Adjacent defects closed

1. An aliased compiler-provided member with a capital letter bound nothing. from collections import Counter as C -> undefined variable (C), while deque as dq worked. PyStdAliasRecord stored the member LOWERCASED and the call table is matched as spelled, so collections.counter resolved nothing. Stored as spelled now; the two value-table readers (PyIsSysStreamAhead, PyParseSysStream) fold it themselves, keeping from os import SEEK_SET working.
2. A builtin type held as a value leaked what it built. pybtype_call0 (f = list; f()) and pybtype_call1 (f("ab"), f([1, 2])) boxed the new object with PyObjAsVar, which takes its own reference, and never dropped the construction's: 1.8 blocks per call for list/set, 3.8 for dict, -dPXX_ALLOC_CENSUS 1000 rounds. Flat now (live 6-10).
3. f = dict; f(d) returned d itself, not a copy (pydict_v hands back the argument's own dict). It copies now, and drops Counter/defaultdict/ OrderedDict modes as dict(x) does.
4. set(), tuple(), frozenset() through a type held as a value raised "not supported yet"; they build the empty value now.

Measured

Census, -dPXX_ALLOC_CENSUS, the full defaultdict exercise in a loop: 100/1000/3000 rounds live 109/134/133. The OrderedDict exercise including an LRU loop: 19/16/48. A discarded d.popitem(last=False) first leaked two blocks per call because the overload forwarded to popitem() and so was not classified as returning what it minted (ClassifyProcResultFresh); it builds its own tuple now, live 10.

Test: test/test_nilpy_collections_defaultdict_and_ordereddict.npy, byte- identical to CPython 3.14 under -dPXX_HEAP_DEBUG and on i386.