We report our Keck/MOSFIRE and Magellan/LDSS3 spectroscopy for an [OII] Blob, OIIB10, that is a high-$z$ galaxy with spatially extended [OII]$lambdalambda3726,3729$ emission over 30 kpc recently identified by a Subaru large-area narrowband survey. Th
e systemic redshift of OIIB10 is $z=1.18$ securely determined with [OIII]$lambdalambda4959,5007$ and H$beta$ emission lines. We identify FeII$lambda$2587 and MgII$lambdalambda$2796,2804 absorption lines blueshifted from the systemic redshift by $80pm50$ and $260pm40$ 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, $250pm140$ 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 OIIB10s star-formation activity. We estimate a mass loading factor, $eta$, that is a ratio of mass outflow rate to star-formation rate, and obtain $eta>0.8pm 0.1$ 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.
