Aha! let me have a stab...any formation mechanism for this interstellar asteroid should account for the coincidence of 'Oumuamua's velocity being so close to the Local Standard of Rest'
The reference says that the initial velocity is closest to the local M Dwarf stars (Red Dwarfs) which are relatively common in the Galaxy.
From Wikipedia - "Many red dwarfs are orbited by exoplanets, but large Jupiter-sized planets are comparatively rare." https://en.wikipedia.org/wiki/Red_dwarf#Planets
So, abundant stars, with many small planets. How about two of these planets smash into each other and a fragment of metallic core blasts off towards us?
(Oh, and the local aliens hop on board for the ride, hollow it out and live there, and watch us and giggle gleefully as we speculate about what it is. )