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"Lóbrego" is an adjective which is often translated as "gloomy", and "oscuro" is an adjective which is often translated as "dark". Learn more about the difference between "lóbrego" and "oscuro" below.
lóbrego(
loh
-
breh
-
goh
)An adjective is a word that describes a noun (e.g., the big dog).
1. (general)
a. gloomy 
El aspecto lóbrego del pueblo abandonado lo hizo el lugar perfecto para filmar la película de terror.The gloomy appearance of the abandoned town made it the perfect place to film the horror movie.
El interior de la iglesia era lóbrego y frío.The interior of the church was gloomy and cold.
b. lugubrious 
Su padre era una persona lóbrega e irascible, a la que yo prefería evitar.His father was a lugubrious and irascible person, whom I'd rather avoid.
oscuro(
ohs
-
koo
-
roh
)An adjective is a word that describes a noun (e.g., the big dog).
2. (complexion)
a. dark 
Ese tinte oscuro de pelo no le sienta bien a su piel clara.That dark hair color doesn't suit her fair skin.
3. (suspicious)
a. shady 
Esa corporación está metida en oscuras negociaciones para controlar los precios.That corporation is engaged in shady business to control prices.
4. (sinister)