The headword -- apparently generated by Aristophanes, Wasps 238: see below -- is a masculine noun in the nominative singular. See LSJ s.v., epsilon 1387, and the following notes for other meanings.
[1] At Aristophanes, Wasps 238 (web address 1), the chorus-leader recounts how the chorus pilfered to\n o(/lmon (the round smooth stone, accusative singular of the headword) one night. One scholion to the passage identifies this as o(/lmon to\ mageiriko/n, a cooking stone; but see below, n.3 (end). The glossing adjective is the neuter nominative/vocative/accusative (and masculine accusative) singular form of mageiriko/s, -h/, -o/n (fit for a cook); see LSJ s.v.
[2] The Pythia, Apollo's priestess at Delphi (OCD(4) s.v. Delphic oracle), employed two kinds of tripod: a three-legged bronze bowl for holding oracular tokens, and a stool upon which she sat while delivering prophecies; cf. pi 3137, theta 281 (end), and Smith, pp. 1014-5.
[3] A scholion (= D scholia) to Homer, Iliad 11.147 (web address 2) is identical, except that it defines a o(/lmos as a koi=los li/qos (hollow stone). Given the text, this is problematic. This verse and its context describe how Agamemnon slew Hippolochus. The Greek king cuts off the arms of his opponent with a sword, beheads him, and then sends the Trojan's armless, headless torso "rolling like a round stone among the throng", to quote the Loeb edition's translation (Murray, p. 503). This rendering of the passage thus combines the scholiast's interpretation of o(/lmos as a stone with the Suda lexicographer's view that it is round, not hollow. Objecting to the very dubious image of a headless, armless human torso rolling about the field of battle like a round stone, Hainsworth (p. 241) suggests that the o(/lmos of Homer's simile signifies a hollowed-out log. He cites Hesiod, Works and Days 423 (web address 3), where the farmer is advised to hew a three-foot mortar (o(/lmon tripo/dhn ta/mnein) during the autumnal woodcutting season, as precedence for this proposed exegesis (Hainsworth, ibid.). Indeed, a second scholion to Aristophanes, Wasps 238 (web address 1), quotes this very line of Hesiod and observes that it could be understood as the mortar being made of wood (w(s culi/nou o)/ntos tou= o(/lmou). Finally, Hesychius omicron595 s.v. o(/lmos notes that this tool, in addition to being a stone for crushing plants, could be a cylinder.