"Yes, good night."

Translation: Ano, dobrou noc.

[lhgates]

Is dobrou spelled with ou because noc is masculine?

[endless_sleeper]

No, noc is feminine.

Dobrou is an accusative form of dobrá. Accusative case is used mainly for objects of a verb.

Look at it as a shortening of Přeji ti dobrou noc. 'I wish you a good night.'

The verb přát něco 'wish sth' is used with accusative.