What does it mean “sechirus lo kania except for in ona’ah” — in general and in context to how the M”A brings it in OC Siman 246 Sif-Katan 8?
This means that a rental contract is not considered as a full sale, in terms of bringing the rented property into the “ownership” of the tenant/renter.
For instance, if a Kohen rents a cow, the cow does not become his “property” for purposes of feeding the cow Terumah, and if a Yisrael rents a cow from a Kohen, the cow can continue to eat terumah.
The exception to this rule is the matter of ona’ah, for which a rental is considered as a sale (the Gemara writes that sechirus is a “daily sale”), so that the laws of ona’ah apply to it just as to a sale. This is the view (that ona’ah is the exception to the rule) adopted by Tosafos in Avodah Zarah (15a) and in Bava Metzia (56b).
Another possible distinction is that sechirus is considered a sale for monetary matters, but not for matters related to issurim. This is possible according to the Terumas Ha-Deshen (210) and Shut Binyamin Ze’ev (164), who apply the concept of sechirus being a daily sale to other matters.
See also Rashba (1, 1028), and see Shulchan Aruch (192:13), among many other sources that can be cited. The topic is too involved to deal with comprehensively in this forum, and the above is only a sampling.