From Middle High German Odere, from Old High German Odera (in Latin texts; also attested as Oddara and various other forms). Further origin unknown, but usually considered Indo-European. Udolph connects it with Avestan 𐬀𐬛𐬎 (adu, “watercourse”), which is ultimately from Proto-Indo-European *wódr̥ (“water”), and further with various eastern European hydronyms (including the Odra in Croatia, the Attersee in Austria, and the Adria). If correct, the initial O- would suggest that the name entered Old High German through Slavic, its earlier Germanic name (unknown but likely from the same etymon) having been lost after the Migration Period.

From Middle High German oder (whence also Yiddish אָדער (oder)), from Old High German odar, an alteration of odo by analogy with wedar (whence modern weder). The form odo is an assimilated, unstressed form of eddo (also edo), itself cognate with Old Saxon ettho, Old English eþþa, oþþe, Gothic 𐌰𐌹𐌸𐌸𐌰𐌿 (aiþþau). This formation is of uncertain origin: While the second component is Proto-Germanic *þau, the first may be *eh, *iþ (cf. Gothic 𐌹𐌸 (iþ)), or *jaba(i) (whence German ob, cf. Dutch ofte).