  The first interstellar object 'Oumuamua is discovered in 2017. When 'Oumuamua
travels in interstellar space, it keeps colliding with interstellar medium
(ISM). Given a sufficiently long interaction time, its rotation state may
change significantly because of the angular momentum transfer with interstellar
medium. Using generated Gaussian random spheres with the dimension ratios 6:1:1
and 5:5:1, this paper explores the ISM torque curve and proposes that ISM
collision may account for 'Oumuamua's tumbling with the simple constant-torque
analytical method. The statistic results show that the asymptotic obliquities
distribute mostly at 0 and 180 degree and most cases spin down at the
asymptotic obliquity, indicating the ISM collision effect is similar to the
YORP effect with zero heat conductivity assumed. Given a long time of
deceleration of the spin rate, an initial major-axis rotation may evolve into
tumbling motion under ISM torque. Using a constant-torque analytical model, the
timescales of evolving into tumbling for the sample of 200 shapes are found to
range from several Gyrs to ens of Gyrs, highly dependent on the chosen shape.
The mean value is about 8.5 Gyrs for prolate shapes and 7.3 Gyrs for oblate
shapes. Rotation of asteroids in the Oort cloud might be also dominated by the
ISM collision effect since the YORP effect is quite weak at such a long
distance from the Sun. Although this paper assumes an ideal mirror reflection
and a constant relative velocity of 'Oumuamua, the results still show the
importance of the ISM collision effect.
