The "anchor" heerarka qeexaya shabakadda kala soocida MK ee xiddigaha taxanaha ugu weyn ee nooca O, i.e. heerarkaas oo aan isbeddelin tan iyo horraantii qarnigii 20aad, waa 15 Monocerotis (O7 V) iyo 10 Lacertae (O9 V). The Morgan<unk>Keenan<unk>Kellerman (MKK) "Yerkes" atlas laga soo bilaabo 1943 ayaa ku taxan heerarka nooca O ee u dhexeeya O5 iyo O9, laakiin kaliya waxay u qaybiyeen iftiinka fasallada O9s. Labada heerarka MK O9 V waxay ahaayeen Iota Orionis iyo 10 Lacertae.
MKK O9 V