Send me an event you think happens in the story of Cú Chulainn's death, and I'll tell you whether it's on Oidheadh Con Culainn (15th century) or Brislech Mór Maige Muirthemne (~9th?? century). Or neither.
The Morrígan tricking him into eating dog meat!
Sort of neither. There is a dog meat incident in both texts; the Morrígan isn't involved, and Cú Chulainn doesn't actually eat it.
In BMMM (§11 in Kimpton's ed.), Cú Chulainn comes across three witches (origins unspecified) cooking a dog. A geis prevents Cú Chulainn from not going towards a cooking fire like this, but also prevents him from eating dog meat. One of the witches (identify unspecified) shames him into joining them and passes him some of the meat. He takes it and places it next to his leg; the hand and leg that touched it are weakened.
In OCC (§30 in Van Hamel's ed.), Cú Chulainn comes across the daughters of Cailitín, among them Badb ingen Cailitín (not to be confused with the other Badb, but it's so easy to do so), cooking dog meat. He turns the chariot away to avoid them, so Badb ingen Cailitín takes the cooking spit and yeets it directly at Cú Chulainn. Cú Chulainn catches it and raises it above his head to throw it back, but as he does so, a drop of blood drips from it onto him. This blood, we're told, is 'poisonous'. He then throws it back and impales Badb with it (wounding, not killing her).
In both texts they are using spits made of rowan wood. Rowan often shows up in conjunction with magic in medieval Irish texts (e.g. in Tochmarc Étaíne it's rowan wood that Étaín is struck with to transform her) so I assume that's presented to flag that it's a supernatural situation, but I have no particular source for that assumption.
BMMM explicitly sets this situation up as a matter of trying not to break one geis and being forced to sort of break another one; OCC doesn't actually use the word geis at any point in this section or say that he's forbidden to have anything to do with dog meat, that's merely presented as an Ominous Detail and it's the 'poisonous' nature of the blood that's the issue. It may be poisonous because of a geis, but since this text usually doesn't hesitate to mention them and there are a bunch more of them than there are in BMMM, it's slightly surprising that it doesn't say that here!
For a detail that shows up in every retelling ever, it's surprising how differently this scene actually goes down in the text.
[Caveat with all of these asks is that I am basing my answers on the manuscripts of OCC edited by Van Hamel and so anything to do with names and terminology is specific to those manuscripts. There are a lot of manuscripts of OCC and it's possible some use slightly different phrasing or names.]
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