There's a consistent theme in Dark Souls about how your obsession with something will change the way you look, whether it is from a seemingly "negative" impulses (greed, gluttony, lust...) or seemingly "positive" emotions (love, devotion, faith, gratitude)
Gaping Dragon was consumed by the design to eat, the Crow Demons were obsessed with Velka, Solaire wanted to become a sun, Moon followers wanted to appear feminine, the Silver Knights followed Gwyn to the Kiln and were burnt and warped, the Egg Bearers, the Covetous Demon, the Abysswatchers - the list goes on.
In this light, I really think interpreting Ornstein's fate along similar lines is the way to go (in my opinion of course). I tend to view the characters stories as part of the broader narrative principle, and in this case it's the theme about how obsession reshapes one’s very being. While Ornstein may not have been the KotS himself, every mention of him in Dark Souls III points toward his seeking out the Firstborn and transforming along the Path of the Dragon.
See, from his very first appearance Ornstein already made himself look like the Firstborn: he wore a golden armor adorned with red, he used a similar spear to the Firstborn's, and his pose for his statue was also similar to the statue in Sunlight Altair. (I don't mention the lion motif here because I still can't decide which one of them claimed that first)
(Btw the other arm of the statue actually bent like Ornstein's)
DS3 only confirmed this devotion further as in they explicitly stated he was faithful to the Firstborn (Sacred Oath description - which also means they likely they have an oath together), he left the cathedral to seek him (his set's description). And most tellingly, we encounter only his armor in the form of a floating orb - an intentional design choice by the developers. If they had planned to imply he simply left it behind it could have been placed in a chest like his own Leo Ring at Anor Londo. If they want to suggest he died in battle there, it should have been found on a corpse. Instead, his armor was discarded - a clear reference to the Path of Dragon ritual.
Taken together, these details strongly support the interpretation that Ornstein's devotion led him to follow his liege's new path, went from a Warrior of Sunlight to a follower of Dragons. His story really feels consistent and fits the overall narrative if you view it this way. This also mirrors the story of the Silver Knights and Gwyn, which is fitting again because well Ornstein was very likely to started a Silver Knight, and Nameless King is the heir of sunlight.
Of course, this is only an interpretation as nothing is concrete. I really don't want this to come across as I'm imposing this as the only truth. In fact I wrote this because I was daydreaming about some scenes where Ornstein resented NK due to many things. But when I look back at Ornstein's design and item descriptions, or at Dark Souls' narrative at a whole, this is the interpretation I've always found most convincing.
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