  At present, there exists no consensus in the astronomical community regarding
either the bulk composition or the formation mechanism for the interstellar
object 1I/2017 U1 ('Oumuamua). With the goal of assessing the merits of the
various scenarios that have been suggested to explain 'Oumuamua's appearance
and observed properties, we report a number of new analyses and provide an
up-to-date review of the current hypotheses. We consider the interpretations
that can reconcile 'Oumuamua's observed non-Keplerian trajectory with the
non-detection of traditional cometary volatiles. We examine the ability of
these proposed formation pathways to populate the galaxy with sufficient
interstellar objects such that the detection of 'Oumuamua by Pan-STARRS would
be statistically-favored. We consider two exotic ices, hydrogen and nitrogen,
showing that the frigid temperature requirement for the former and the
necessary formation efficiency of the latter pose serious difficulties for
these interpretations. Via order-of-magnitude arguments and hydrodynamical
cratering simulations, we show that impacts on extrasolar Kuiper Belt analogues
are not expected to generate N2 ice fragments as large as 'Oumuamua. In
addition, we discuss observational tests to confirm the presence of these ices
in future interstellar objects. Next, we examine the explanations that
attribute 'Oumuamua's properties to other compositions: ultra-porous dust
aggregates and thin membranes powered by solar radiation pressure, among
others. While none of these hypotheses are perfectly satisfactory, we make
predictions that will be testable by the Vera Rubin Observatory to resolve the
tension introduced by 'Oumuamua.
