noun
main sweet served with thick tea
In the tea ceremony, the sweet eaten with koicha (thick tea), as opposed to the lighter sweet served with usucha (thin tea).
See also: 濃茶
濃茶の前に主菓子をいただきます。
We have the main sweet before the thick tea.
From 主 (omo, 'main') + 菓子 (kashi, 'sweet'). The reading おもがし is a standard compound reading.