Polite term for a side dish, often used in the context of a traditional Japanese meal. The plain form is 菜 (さい).
See also: 菜
What is today's side dish?
Would you like some more of the side dish?
Polite prefix お (o-) + 菜 (sai, 'side dish'). The kanji 菜 originally refers to vegetables or greens, but in this compound it broadly means a dish accompanying rice.