There is no way to get the list of users under a single OU (and only those users). However, you can just grab the full list of users in the Google Apps instance along with their OU using a call to:
https://www.googleapis.com/admin/directory/v1/users?customer=my_customer&fields=nextPageToken%2Cusers(orgUnitPath%2CprimaryEmail)
based on Vic Fryzel's presentation at I/O, I believe Google plans to continue to improve the capabilities of the query parameter, eventually allowing a query based on OU (as well as other query parameters).