Yu-Gi-Oh! » Rulings Q&A

If Cold Wave resolves while Royal Oppression is already face up on the field, can Royal Oppression be activated that turn?

No

Royal Oppression in this case is already face up on the field. Cold Wave says you and your opponent cannot play or set any spell/trap cards, but this is using an effect of a trap card that is already face up on the field, which is perfectly legal

https://prnt.sc/135fj4y