Derived as a combination of accusative / object particle を (wo, modern pronunciation o) + the conjunctive form して of classical verb す (su), modern verb する (suru, “to do”, emphasis on active voice). Used to mark the indirect object or patient of a causative expression. This appears very early in the written record in the Shoku Nihongi, completed in 797 CE, representing late-Nara period and early-Heian period Japanese. This usage persists in the modern language, but is restricted to formal literary writing.
Historically used primarily in the Japanese reading of 漢文 (kanbun, literally “Chinese texts”; Japanese texts written in a version of Classical Chinese). Later, in works written natively in Japanese, the を (o) was omitted and just して (shite) was used instead. Over time, をして (o shite) with the を (o) again became common, and in the modern language, this is used exclusively in literary contexts to impart a traditional and formal tone.
Prior to the mid-Heian period, the verb was not necessarily conjugated in the causative, and をして (o shite) was used to mark the subject of the action. Starting from the mid-Heian period, the verb was conjugated with the causative auxiliary しむ (shimu) in the classical form or しめる (shimeru) in the modern form.
In modern non-literary Japanese, this has been superseded by the particle を (o) for intransitive verbs (自動詞, jidōshi), and に (ni) for transitive verbs (他動詞, tadōshi), together with the standard causative endings せる (seru) or させる (saseru).