noun which may take the genitive case particle 'no'
open for business; now open
Typically used on signs at the entrance of a shop or restaurant to indicate it is currently open.
There was a sign saying 'Open' in front of the shop.
Please feel free to come in whenever we are open.
Compound of 営業 (business, sales) + 中 (in the state of). The reading is standard on'yomi.