The surname Ostan is a surname typical of eastern Veneto, especially of the area situated between Portogruaro and San Stino di Livenza, in the zone of historical and linguistic contact between Veneto and Friuli, where its oldest and most documented origin is concentrated.

From the etymological point of view, the most consistent interpretation considers Ostàn to be an aphetic surname, that is, formed by the loss of the initial vowel, derived from the adjective Agostano, linked to the month of August. This type of adjectival surname was usually applied:

• to people born in the month of August, or
• to individuals connected with seasonal work or activities proper to that period.

This explanation is supported by different authors and is reinforced by the coexistence, in the same geographical area, of clearly related surnames such as Agost, Agosta, Agosti and Agosto, which share the same chronological and semantic base.

In this framework, Ostan represents the abbreviated and oldest form, fixed locally in the Veneto-Friuli area.

From Ostan developed direct morphological derivatives, by means of suffixes characteristic of the area:

• Ostanèl / Ostanello: diminutive and patronymic forms, created with the suffixes -èl and -ello, very productive in eastern Veneto and Friuli. These variants do not indicate a different origin, but a family ramification or an internal differentiation between branches of one and the same cluster.
• Ostanetti / Ostanini (where documented): pluralized or patronymic forms, arisen to indicate descent from or belonging to a family group originating in an Ostan.

All these surnames share the same etymological origin and must be interpreted as internal developments of one and the same onomastic base.

Some authors, such as E. Rossoni, further mention an alternative hypothesis, according to which Ostan could be linked to an old personal name of Celtic origin, Drostan (from which Tristano also derives). This possibility is not excluded, but it is considered secondary to the aphetic explanation from Agostano, especially owing to the strong geographical and morphological coherence of the group of variants.