The meaning of "o' ṃ" is quite literally "I am existence" and it is used at the end of the invocation to the god being sacrificed to (anuvakya) as an invitation to for that God to partake of the sacrifice.
The meaning of "o' ṃ" is quite literally "I am existence" and it is used at the end of the invocation to the god. This god is being sacrificed to (anuvakya) as an invitation to for that God. This was to partake of the sacrifice.