Conjurations are not creatures, they are effects with the Conjuration keyword. You can't OA a Flaming Sphere, nor a Mage Hand. The -exact- same logic applies here. Only creatures can trigger OAs.
Powers with the Summoning keyword DO bring forth creatures, and that's where the confusion lies. Spirit Companions do NOT have the Summoning keyword. Really. Read the power.
I don't have my books with me, so can you confirm this? Are OAs really restricted to creatures? Or do Flaming Spheres/Mage Hands not draw OAs not because they are conjurations, but because they simply can't be targeted by attacks in the first place (again, because they're conjurations)?
Unless OAs spell out a "must be a creature" stipulation, I would think spirit companions, being targetable, would be vulnerable to OAs. However, as I'm thinking of rolling a Shaman very soon (come on CB, update!) I can't say I'm not open to being convinced otherwise!

