There are multiple functions that operate on ObjectIDs in libbson that have unsafe variants:

bson_oid_compare_unsafe
bson_oid_equal_unsafe
bson_oid_hash_unsafe
bson_oid_copy_unsafe
bson_oid_init_from_string_unsafe
bson_oid_get_time_t_unsafe

The unsafe variants omit null checks and are marked inline. They are undocumented.
The "safe" variants just check that arguments are not null before calling the unsafe variant.