  We report our Keck/MOSFIRE and Magellan/LDSS3 spectroscopy for an [OII] Blob,
OIIB10, that is a high-$z$ galaxy with spatially extended
[OII]$\lambda\lambda3726,3729$ emission over 30 kpc recently identified by a
Subaru large-area narrowband survey. The systemic redshift of OIIB10 is
$z=1.18$ securely determined with [OIII]$\lambda\lambda4959,5007$ and H$\beta$
emission lines. We identify FeII$\lambda$2587 and MgII$\lambda\lambda$2796,2804
absorption lines blueshifted from the systemic redshift by $80\pm50$ and
$260\pm40$ km s$^{-1}$, respectively, which indicate gas outflow from OIIB10
with the velocity of $\sim 80-260$ km s$^{-1}$. This outflow velocity is
comparable with the escape velocity, $250\pm140$ km s$^{-1}$, estimated under
the assumption of a singular isothermal halo potential profile. Some fraction
of the outflowing gas could escape from the halo of OIIB10, suppressing
OIIB10's star-formation activity. We estimate a mass loading factor, $\eta$,
that is a ratio of mass outflow rate to star-formation rate, and obtain
$\eta>0.8\pm 0.1$ which is relatively high compared with low-$z$ starbursts
including U/LIRGs and AGNs. The major energy source of the outflow is unclear
with the available data. Although no signature of AGN is found in the X-ray
data, OIIB10 falls in the AGN/star-forming composite region in the line
diagnostic diagrams. It is possible that the outflow is powered by star
formation and a type-2 AGN with narrow FWHM emission line widths of $70-130$ km
s$^{-1}$. This is the first detailed spectroscopic study of oxygen-line blobs,
which includes the analyses of the escape velocity, the mass loading factor,
and the presence of an AGN, and a significant step to understanding the nature
of oxygen-line blobs and the relation with gas outflow and star-formation
quenching at high redshift.
