Is it possible to take an excerpt from the first rich text area in a matrix?
You can filter by block type: http://buildwithcraft.com/docs/matrix-fields#filtering-by-block-type

In short:

{% set ogDescription = false %}

{% for block in entry.myMatrixField.type('richText').limit(1) %}
  {% set ogDescription = block.text | striptags | slice(0, 155) %}
{% endfor %}

{% if ogDescription %}
  ...
{% endif %}


Also, this was suggested in the comments:

{{ entry.myMatrixField.type('richText').first() | striptags | slice(0, 155) }}