OStatus is a collection of protocols, which was known for powering StatusNet (now known as GNU Social) since 2010.

ActivityPub is a lot more recent, and is a "consolidated" protocol designed to replace OStatus. It's fairly recent, so Mastodon is one of its first implementations, and Gargron (the main dev of Mastodon) contributes to the W3C Working Group in charge of the spec since ActivityPub support was added to Mastodon.