Opal Lounge comes with particular rooms (the level below those with Club Lounge access). This can be found in the "Room Details" when booking; for example the Premium Room comes with Opal access. Outside of "happy hour" etc. it is also open for all residents if they pay.

However, the Opal Lounge is not open all year round, so check with the hotel to avoid disappointment as they don't make this clear when booking.