There are two questions tagged mtg-online; however, the name of the game is Magic Online, not Magic: The Gathering Online (the "The Gathering" part is just part of the logo in this case: it is not referred to as such in copy).
This solution would also have the side effect of matching our usage of Magic 2012/2013, standardizing Magic-related games on
magic-* for the main tag and
mtg-* as a synonym.
Rilgon Arcsinh brings up the valid concern of whether the change could adversely affect our Google ranking for those who might be searching under other forms of the game name (including, ostensibly, "MTG Online"). This change will have very little (if any) effect on this:
Google silently auto-expands "mtg" to "magic the gathering" in searches, and searching for "mtg online" alone is dominated with results for "magic online".
Because of Google's silent expansion of Magic-related terms to "magic the gathering", we already rank first even when we don't use one of the colloquial forms: for example, this question comes up as the first result for "mtgo effective card collection" even though "MTGO" doesn't appear anywhere in the question.
That is to say, Google is smart enough to understand you mean Magic Online when you try to search on "mtgo", "magic online", "mtg online", or "magic the gathering online", so it's unnecessary to explicitly mention the colloquial forms in our questions or tags. The synonyms are then just needed in case someone starts typing "mtg" instead of "magic" when they go to add tags.