Stands for old married Spirk (Spock/Kirk). A popular ship inspired by Star Trek the movie series (TMS). It is considered canon by many fans.
OMS is generally regarded with affection because:
1. It isn't about the attractiveness of the characters
2. TMS (the movie series) is brilliant (c'mon, they travel back in time to save whales)
3. The fanfics tend to be fluffy/comfort fics and may also incorporate many deeper or more serious topics.
Person 1: OMS is my OTP.
Person 2: But why would you ship old people?
Person 1: how dare you.