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HD 50138 is one of the brightest B[e] stars and is located at a distance of ∼380 pc with strong infrared excess. The star was observed in [O i] 63 μm and [C ii] 158 μm with high velocity resolution with upGREAT on SOFIA. The velocity-resolved [O i] emission provides evidence for a large gas disk, ∼760 au in size, around HD 50138. Whereas previous interferometric observations gave strong evidence for a hot gas and dust disk in Keplerian rotation, our observations are the first to provide unambiguous evidence for a large warm disk around the star. Herschel/PACS observations showed that the [C ii] emission is extended, therefore the [C ii] emission most likely originates from an ionized gas shell created by a past outflow event. We confirm the isolated nature of HD 50138. It is far from any star-forming region and has low proper motion. Neither is there any sign of a remnant cloud from which it could have formed. The extended disk around the star appears to be carbon-poor. It shows OH and [O i] emission, but no CO. The CO abundance appears to be at least an order of magnitude lower than that of OH. Furthermore, 13CO is enriched by more than a factor of five, confirming that the star is not a Herbig Be star. Finally, we note that our high-spectral-resolution [O i] and [C ii] observations provide a very accurate heliocentric velocity of the star, 40.8 ± 0.2 km s−1. [ABSTRACT FROM AUTHOR]