Is this buckthorn?

It could be. But there are many plants with oval leaves and toothed margins. You've given us a single photo of one leaf.

Read here about identifying buckthorn:
http://www.ci.burnsville.mn.us/DocumentCenter/Home/View/2164

Here are some quick identifiers. First, and most important this time of year, buckthorn is among the very last to lose their leaves. The leaves remain green deep into autumn. Second, look for the persistent berries. Third, the leaf attachment is semi-alternate: not quite opposite and not quite alternate.