olv.portal.miitoo was a Miiverse glitch discovered in early 2014 by a user by the name of Ethan (MortifiedPanda). This glitch caused the Yeah button for all posts affected by it to display as [olv.portal.miitoo.] rather than the normal [Yeah!], although it wasn't well-known until users such as Robert and Loid helped aid its spread. However, it has since been patched.
Feeling IDs go from 1 to 6 and determine what emotion your post uses. Usually, you can only submit a post with feeling IDs 1-6, but there is an unused feeling ID that was in Miiverse since its beginning, feeling ID 8. You used to be able to post with this.
It works, however the yeah button appears as "olv.portal.miitoo."; note the full-stop at the end of the string.
This is because every string Miiverse has (such as 'Post', 'Comment', 'Community', etc) is translated into every language Miiverse supports. The Yeah button text, in this case, is referred to as olv.portal.miitoo, with the feeling ID at the end, so for example, for feeling ID #1, it would be "olv.portal.miitoo.1". The problem is that there is a value for olv.portal.miitoo.1, but there is no value for olv.portal.miitoo.7, so Miiverse resorts to showing the original text instead of the adapted, translated text.