The Linaro project is backed by Canonical (the people behind Ubuntu) among others which means this is a great place to start if you want to run Ubuntu on your Overo COM.

Its monthly release includes developers and community builds that support OMAP3 devices. The Getting Started Guide outlines the installation process using the pre-built images. However if you would like to customize the hardware pack, take a look at linaro-overo-config and ubuntu-kernel-ci.