Likewise in other lexica; references at Photius omicron420 Theodoridis.
The spelling o)pta/nion occurs in comedy, but o)ptanei=on would be expected from o)ptanh/i+on (cited by ps.-Herodian and the Etymologicum Magnum) as well as o)ptaneu/s "one who roasts."
There seems to be no connection with omicron 487 (q.v.).