Bilbo complains after a high-flavoured journey through amazing treasures and tunnels – and the OED calls this an imitative word.  We include it in our list of onomatopoeic words and give it a “low” tag.

• 13.050 But, ow! this wind is cold!’

“ow, int.” OED Online. Oxford University Press, March 2015. Web. 20 May 2015.