Rashi on Leviticus 4:23/רש"י על ויקרא ד׳:כ״ג

1או הודע is the same as “IF” (אם‎) [THE THING] WAS MADE KNOWN TO HIM — There are many passages where או is used in the sense of אם, and again where אם stands in the place of או. A similar instance is: (Exodus 21:36) “או נודע כי שור נגח הוא”, which means “if it was known that the ox was wont to thrust” (cf., however, Rashi on that verse and our Note thereon).

2הודע אליו IT WAS MADE KNOWN TO HIM — when he committed the sin he was under the belief that it was something permissible, afterwards it became known to him that it was a forbidden thing.