The wolf ring outside of the grave area is intriguing. I tend to believe that the reference to bandits suggests that at one point someone almost got away with something, got lost in the woods and met their end. I don't have a lot of evidence beside the bandit reference. Ciaran's ring is also intriguing. I tend to to believe O&S for several reasons but don't rule out Ciaran. I think he might be a third party in the deed but feel more confident about O&S. Smough's lore makes me believe he aspired to be the fifth Beatle (knight) but his bloodthirsty ways prevented it. But who ruled him out of the group? Ornstein doesn't seem bothered by Smough in the least. Ciaran is pretty much missing in the game, as is Gough. Evidence suggests that Gough stayed in Anor Londo as he led the Great Archers who still have a presence and his ring is still there as well. I'll get to my Ciaran theory in a second. So in order to be excluded from the group of knights there would have to be either a vote that didn't go in his favor (either split or majority) or there was a leader who ruled him out. In the event of a vote, I feel confident Artorias said "no thanks." Either he was the leader and his vote was law or one or more other Knights agreed. Ornstein seems to like Smough and their Statues grace the entry to Anor Londo seeming to suggest they became equals at some point in time. Statues of the other knights are NOT present seemingly. Gough seems to be around like I said but is no longer revered. Perhaps he voted "no" to Smough as well but recanted once Artorias and Ciaran were gone. Or he voted "yes" but was relegated to a lower status. Or they kept him around for his value as a tactician. I feel strongly that Gough was around but diminished. Ciaran is harder to peg. The presence of the ring makes me confident he was at the grave. I am pretty sure he didn't leave either. A few theories (with little proof). Assuming Artorias was the leader, perhaps there was resentment and Ciaran went along with O&S (and possibly Gough) to remove Artorias from power. This is definitely possible as they seem to have served Gwyn and Artorias joined the Darwraiths (a covenant Gywn went to great lengths to destroy). Adding to this is Ciaran's apparent specialty.....backstabs. This could be taken to mean not the move but the act of treason. I do believe that Ciaran might have backstabbed Artorias. However, I don't think that Artorias died in the forest. The presence of the sword and Sif alive seem to suggest that the body was moved after death as I don't think that Sif would be left alive or the sword left as tribute (at least not if O&S were involved there). I believe that the ring, like the body got there after the murder of Artorias. Did Ciaran feel guilty about his involvement and leave the ring as tribute? Perhaps he sent the ring via a messenger as I believe it's found on a corpse smaller than the knights are suggested to be. This could be perhaps another bandit who didn't quite make it but either way the ring was there. This could have been done covertly to avoid leading O&S to Artorias' grave. Ciaran would have been watched with my theories as I don't think he was fully on board with Smough as the fifth knight and a visit to the grave would be bad news. Or did Ciaran make a stand with Artorias and also paid the price? Backstab meaning he betrayed Gwyn. The lack of a second grave or lore to that effect makes me doubt this.
To make my case for O&S as the murderers, EVERYTHING about Artorias makes me believe he is special (more so than Rhea and Solaire) and therefore likely pretty stellar combat wise. O&S are definitely buddy-buddy which goes against the concept of Smough not being invited to the Knight party. They have practice fighting together which I think they would need to do to beat Artorias (they would have lost if Sif was there unless Gough and/or Ciaran joined in). Anor Londo has been renovated to remove the old heroes (Gwyn's first born, Ciaran, Gough and Artorias). Ornstein and Smough are the new idols meaning they rose to power together (possibly after Gwyn took off but I doubt it).
So to summarize my beliefs......Artorias and his divine weapon was originally assigned to hunt down evil forces but eventually realized he didn't agree with the way things were heading with Gwyn. Knowing that his actions would be considered treason and punishable by execution he bequeathed his weapon to Sif for safekeeping and left, on his own down his new path. Artorias joined the Darkwraith covenant and his cursed sword was forged as a replacement to his divine weapon and a symbol of his new journey. Fearing the covenant now that the best knight was on board, Gwyn ordered it's destruction. With Artorias out of the picture, Ornstein became the new head of the knights, initiated Smough to the ranks. Gough and Ciaran were torn but reluctant to leave their old life behind. They remained with the knights but were demoted in the new regime. New Londo was flooded in the battle and Artorias was murdered in a final stand. Possibly this occured in New Londo (Ornstein has a LOT of drake heads which aren't big enough to have been a trophy of the war between the Lords and the Dragons). I am unsure if Ciaran was an accomplice but lean toward "probably." Eventually, those loyal to Artorias (Sif, possibly Alvina, possibly Ciaran) recovered his body and hid it in the forest, again the only place I feel that has true "life" in Lordran.
Sorry for the thesis