Interstellar Origin:

Unlike most objects observed in our solar system, 'Oumuamua does not orbit the sun. It entered our solar system from interstellar space and is now leaving.

Elongated Shape:

'Oumuamua is significantly more elongated than typical asteroids and comets. Its shape is estimated to be cigar-like, with a length-to-width ratio of about 10:1.

Speed and Trajectory:

'Oumuamua's speed and trajectory were unlike anything seen before, further supporting its interstellar origin.

Lack of a Coma:

Unlike typical comets, 'Oumuamua did not develop a coma (the gaseous envelope surrounding the nucleus of a comet) as it approached the sun.

Unexpected Acceleration:

'Oumuamua exhibited a small but noticeable acceleration not solely attributable to the sun's gravity. This led to speculation about potential outgassing or other non-gravitational forces.

(http://www.autoadmit.com/thread.php?thread_id=5760647&forum_id=2,#49171216)