What was Ornan doing when he turned and saw the angel of the LORD?

Answer: Threshing wheat

Ornan was threshing wheat when he saw the angel, and his four sons with him hid themselves. David then bought the threshing floor.

“Now Ornan was threshing wheat when he turned and saw the angel; and his four sons who were with him hid themselves.”— 1 Chronicles 21:20 (BSB)