Your OAMM membership starts the same day you sign up.

If you signed up for a monthly membership your membership will renew on the same day of the month as the month you purchased it (ie. if you purchase on January 3rd it will renew on February 3rd).

If you signed up for a yearly membership your membership will renew on the same date the following year (i.e. if you purchase on January 3rd, 2015 it will renew on January 3rd, 2016).