Short answer: Its observed speed is greater than the escape velocity of the Solar System.
As of Nov. 20, 'Oumuamua is travelling about 38.3 km/s relative to the Sun. Its location is approximately 124 million miles (200 million kilometers) from Earth [...] though its outbound path is about 20 degrees above the plane of planets that orbit the Sun. The object passed Mars's orbit around Nov. 1 and will pass Jupiter's orbit in May of 2018.
This passage doesn't make it absolutely clear, unfortunately (the author is unnecessarily holding back known facts that would make the answer obvious), but it looks very likely that the author is saying that, at a distance of roughly 1 A.U. from the Sun, the object had a velocity which was a multiple of the escape velocity at the Earth's orbit (namely: ca. 42 km/s).
That means that the Sun's gravity will be unable to hold it.
The only other theoretically possible answer would be if the object had swung by (very, very closely) several gas / ice giants (Neptune, Uranus, Saturn, Jupiter) in succession, and thus gotten several "gravity assists" - something which NASA can accomplish only with modern computers and mid-course adjustments to the trajectories of its space probes.
Regards,
Thank you. Very enlightening, yet not definitive. The velocity may overcome the Sun’s gravity but the Sun also has trajectory as does this object. Velocity is not the sole variable in the equation and the Sun is not a fixed object.
Also telling is the confusion in classification.
I would grill the astronomists further, just to keep them on edge.
And note, NASA PR people are so quick to get a piece of sensational news printed. Remarks such as “It could be interstellar and if it is, it’s the first we’ve ever recorded” by an astronomist becomes “It’s interstellar and is the first ever recorded” by a PR writer.