Poor Ornstien

Those sound pretty nice, it's a shame Ornstein doesn't make a single appearance at all.

The problem with summoning Ornstein for the Kalameet fight would be that it'd either be easy mode, or run counter to the lore of Dragon Slayer Ornstein. However, co-oping with Ornstein for the Artorias fight would make a lot of sense. Orstein could be sent/set out on his own to destroy a corrupted Artorias. How 'bout a special soap stone for setting a sign specifically for Ornstein to summon you like the black eye orb for invading Lautrec? I think that'd be pretty cool. Of course, you'd need Artorias gone from your world too. They could make it so you can't beat Artorias until you help Ornstein and then you can summon Orstein for Artorias.

I put Ornstein into my DLC.

I cosplayed as him once.

How bad must Ornstien have felt when he learned his fellow knights and potential friends all died at Oolacile when he could have helped them or even died alongside them? Nobody thinks about that.

Isn't the whole thing with Ornstein that he's stuck guarding the illusion of Gwynevere (even though He don't know it's an illusion) together with Smough?

When Oolacille fell, Gwyn had already gone to the First Flame to kindle it, and it's probably quite safe to assume that the gods had fled Andor Londo already.

So Ornstein couldn't go to Oolacille to help out, because he had guard duty with Smough, probably appointed by Gwyndolin. No matter whether he was a friend with the other knights or not, he couldn't disobey a direct order.