Ordo Equitum Solis (abbreviated OES) are an alternative music band consisting of French Leithana, pseudonym of Nathalie Herrera (vocals, keyboards, zither, percussions) and Italian Deraclamo, pseudonym of Paolo De Marco (vocals, guitar). The duo formed in 1988, and released albums and toured between 1990 and 2000. In 2001 the band suddenly broke up, but there was a reunion in 2010.

The musical style of OES does not fall into a precise category. While the early albums have pronounced industrial, esoteric darkwave and a touch of medieval influences, with time their musical style matured, embracing multi-faceted sonorities between neofolk, experimentalism and ballad songs. The albums, that include also instrumental tracks, are characterized by a significant switch and mix of genres, making Ordo Equitum Solis' music unusual, various, intimistic and hard to describe. Distinguishing features are Leithana's voice, Deraclamo's folk guitar, lyrics in various languages (English, French, Latin, Italian), highly-emotional melodies, experimental effects, romantic and decadent atmosphere, and remarkable, sometimes provocative arrangements.

Ordo Equitum Solis' releases had often been concept-albums, inspired by symbolic themes or personal experiences. The albums denote also the duality of the band, between Leithana's often inflexible creative genius, and Deraclamo's instinctive inspiration.