Oumuamua wasn't a "traditional" spaceship but it is a solar sail.
https://arxiv.org/pdf/1810.11490.pdf
This point of view is proposed by reearchers Shmuel Bialy and Prof.
Abraham Loeb.
Oumuamua would be then, a thin (1mm) sheet designed to reflect as much
as possible solar radiation. This would explain the acceleration that we
see as it leaves us.
[Moderator's note: They discuss a "width" of 0.3--0.9 mm for the sheet.
I would use "thickness" here. -P.H.]