Dear Rabbi Kornfeld:
On Kritot 23a, concerning a kidney which is Notar and its Heleb, the gamara says there is an "Issur Olin", and therefore Notar should not apply.
Now the "Issur Olin" for Notar is only b'd'eved, since lechatchila there is a mitsvah to burn notar! Why then quote Olin instead of Sreifa, which is lechatchila?
It surely couldn't be because Sreifa is a Mitsvat Asei [so we couldn't say 'Issur hal al issur']: After all, Olin is also an Asei!
Thanks for the help,
Yeshayahu HaKohen Hollander
The Isur of Olin, meaning that one must burn this on the Mizbe'ach, was the first Isur which this Korban encountered. Hence, in a discussion of Ain Isur Chal Al Isur, this would be the first Isur noted. [The Aruch la'Ner asks a similar question, saying that it would seem there is no longer any Isur Olin, as now there is an obligation to burn it because of Nosar! However, he proves from a different Gemara that the prohibition remains despite its becoming Nosar.]