  The first known interstellar object 'Oumuamua exhibited a nongravitational
acceleration that appeared inconsistent with cometary outgassing, leaving
radiation pressure as the most likely force. Bar the alien lightsail
hypothesis, an ultra-low density due to a fractal structure might also explain
the acceleration of 'Oumuamua by radiation pressure (Moro-Martin 2019). In this
paper we report a decrease in 'Oumuamua's rotation period based on ground-based
observations, and show that this spin-down can be explained by the YORP effect
if 'Oumuamua is indeed a fractal body with the ultra-low density of $10^{-2}$
kg m$^{-3}$. We also investigate the mechanical consequences of 'Oumuamua as a
fractal body subjected to rotational and tidal forces, and show that a fractal
structure can survive these mechanical forces.
