Life seems but a quick succession of busy nothings

I have a rather limited definition of Mary Sue, in that for me it inherently implies a negative title (a character must be a cliched self-insert for me to consider them a Mary Sue). There are no good or well-written Mary Sues in my mind, they are all annoying and/or cliched and/or poorly developed. In that vein, a character does not necesarily have to be 'original' to be considered one - there are plenty of Hermione/Ginny/Lily-Sues out there. If, on the other hand, she is well written, realistic, has a purpose outside of sexing up the author's favorite male character, etc, well then I consider her a legitimate original character. These are pretty rare (especially with the instant stigma attached to any female original character. Most of the good writers out there don't bother with OC at all). The only examples of legitimate original characters/non-Mary Sues that I can think of are Cassie's Rhysenn from the DT, or Lori's Allegra from PoU. So, no, an OC doesn't immediately imply a Mary Sue, at least not for me.