You can add messages to be displayed during the video using overlays. You can add an overlay in the studio using xml:
<ooyala>
<overlays>
<overlay start="39" end="45"><html>Thats <b>Chris!!</b> one of our colleagues!</html></overlay>
<overlay start="4" end="15"><html><a href='http://www.edx.org'>Welcome</a> to <b>our course</b>!</html></overlay>
<overlay start="16" end="20">This is raw text</overlay>
</overlays>
</ooyala>