I haven't met this particular phrase, but imagine it is connected with the term 'oxbow lake' - Oxbow lake - Wikipedia, the free encyclopedia . Note the satellite photo of Songhua River.
"Dupree ran his fingers through thin black curls, wondering again if he'd have his father's oxbow hairline by the time he was forty-five"
I get the general idea of this description to mean hair baldness. Could someone explain the use of the word oxbow in this case or even paste me a link with a picture of someone that has a 'oxbow hairline'.
I agree - it is related to a receding hairline which is often curved.
I had never heard this expression either, but it brings an instant picture to mind. An oxbow is a looping bend in a river that eventually leads to the formation of a lake.
If you go to Google Images and type in 'oxbow', you will see the shape. Then type in 'baldness patterns' and you will probably be able to imagine what the author means. He is describing a common baldness pattern in which hair is lost in the temporal regions, leaving an 'oxbow' of hair at the center of the forehead.
I hope this is helpful,