“Oar” now occurs less frequently, though not by much, than its plural per the Google Ngram Viewer.  Before the mid-eighteenth century, this was not the case.  Predecessors of the word seem to come from Scandinavian tongues.

• 08.027 There aren’t any oars.
• 10.009 and oars were pulled,
• 10.045 The white oars dipped

“oar, n.” OED Online. Oxford University Press, March 2015. Web. 9 May 2015.