Who was the very first open-air preacher in the Bible?

Val got it right! The very first open air preacher was… GOD!

“And God said…” (Genesis 1:3)

In fact, he was the very first open NO-AIR preacher. Then when He created the air, He preached! And boy what a sermon…

The heavens declare the glory of God;
the skies proclaim the work of his hands.”
(Psalm 19:1)