This name-form appears in the Etymologies s.v. OT- (OTOS, OTOK) seven (LR 379) as a Noldorin translation of the name Otselen (q. v.) but it is not used in any of the stories referring to the constellation.
The elements of the name are odog 'seven' and gil 'star'.
LR = The Lost Road and other Writings (The History of Middle-earth, vol. 5).