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The first interstellar object 'Oumuamua was discovered in 2017. When 'Oumuamua travels in interstellar space, it keeps colliding with the interstellar medium (ISM). Given a sufficiently long interaction time, its rotation state may change significantly because of the angular momentum transfer with the interstellar medium. Using generated Gaussian random spheres with 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° and most cases spin down at the asymptotic obliquity, indicating that the ISM collision effect is similar to the Yarkovsky–O'Keefe–Radzievskii–Paddack (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 gigayears to tens of gigayears, highly dependent on the chosen shape. The mean value is about 8.5 ± 0.5 Gyr for prolate shapes and 7.3 ± 0.4 Gyr for oblate shapes. Rotation of asteroids in the Oort cloud might also be 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. [ABSTRACT FROM AUTHOR]