Below is a file in Pixelmator with two white circles, one transparent circle and a black background. If put above a video in FCP X, only the part of the video under the transparent circle will show.

Note: I had to research a bit how to do this, particularly how to cut a "whole" in the black background. As I expected, it is not hard.
Summarily:
1) Make three copies of the white circle and position then on the frame.
2) Add a black background by creating a new layer and using the paint-bucket tool to fill it completely. Move it below in the layer order so you see the white circles again.
3) Command-click the middle circle thumbnail in the layers list (this is the part I had to look up and it is SO useful... Thank you for setting me up to find this!). This creates a SELECTION matching the circle.
4) Select the black layer in the list and press DELETE. This creates the "hole" in the black layer
5) Select the middle circle and make it transparent by changing the fill and stroke to None, or disable its layer altogether.