The Spanish word 'oscuro' meaning 'dark' comes from the Latin word 'obscurus', which also meant 'dark' or 'hidden'. The Latin word 'obscurus' itself was formed using the prefix 'ob-' meaning 'against' or 'facing', suggesting the idea of something being blocked from view or facing away from light. Over time, as Latin evolved into Spanish, 'obscurus' simplified to 'oscuro', dropping the 'b' and 'c' sounds while maintaining its core meaning of darkness.

A common related Spanish word is 'oscuridad' (darkness), which is directly derived from 'oscuro'. Another related term is 'oscurecer' (to darken), the verb form that describes the process of something becoming dark.

English speakers might recognize the connection between 'oscuro' and the English word 'obscure', which both come from Latin 'obscurus'. While Spanish 'oscuro' primarily means 'dark' in a literal sense, English 'obscure' has evolved to mean both 'dark/dim' and 'difficult to understand' or 'hidden from view'. Other related English words include 'obscurity' and 'obscuration'.