Board Thread:General Discussion/@comment-25926288-20170508005919/@comment-26620090-20170508183926

Zalvukalic wrote:

Why was Zelena singing. isn't Oz a different realm, not the same kingdom It is.

Out-of-universe reason: Rebecca wanted to sing.

In-universe reason: Zelena was probably singing because she was really happy and wanted to emulate what was happening in the EF.