  We present late-time optical spectroscopy and X-ray, UV, and optical
photometry of the nearby ($d=214$ Mpc, $z=0.0479$) tidal disruption event (TDE)
ASASSN-15oi. The optical spectra span 450 days after discovery and show little
remaining transient emission or evolution after roughly 3 months. In contrast,
the Swift and XMM-Newton observations indicate the presence of evolving X-ray
emission and lingering thermal UV emission that is still present 600 days after
discovery. The thermal component of the X-ray emission shows a unique, slow
brightening by roughly an order of magnitude to become the dominant source of
emission from the TDE at later times, while the hard component of the X-ray
emission remains weak and relatively constant throughout the flare. The TDE
radiated $(1.32\pm0.06)\times10^{51}$ ergs across all wavelengths, and the UV
and optical emission is consistent with a power law decline and potentially
indicative of a late-time shift in the power-law index that could be caused by
a transition in the dominant emission mechanism.
