The syllable Aum (ओ३म्) is rendered by the Arya Samaj using pluti.
Pluti is the term for the phenomenon of overlong vowels in Sanskrit; the overlong vowels are themselves called pluta. Pluta vowels are usually noted with a numeral "3" (indicating a length of three morae), ā3, ī3, ū3, ṝ3, ḹ3, also e3 (ā3i), o3 (ā3u).
Pluta vowels are recorded a total of 3 times in the Rigveda and 15 times in the Atharvaveda, typically in cases of questioning and particularly where two options are being compared. For example:
RV 10.129.5d adháḥ svid āsî3d upári svid āsī3t "Was it above? Was it below?"
AV 9.6.18 idáṃ bhûyā3 idâ3miti "Is this larger? Or this?"