Cleaning an opal ring is a two-step process, since the stone may be porous but the ring itself is likely made of silver or gold. Opals are typically categorised into three levels: solid, doublet and triplet. A solid opal is exactly that--solid--while the doublet and triplet are far more porous and could be damaged if not cleaned correctly.
Prevent damaging your opal ring when cleaning it by never having it cleaned in an ultrasonic jewellery cleaner. The ultrasonic waves can cause minuscule cracks in the stone that can allow moisture inside, which can potentially damage the ring.
Don't soak your opal in anything, as the moisture can seep inside this way as well.
Clean the metal portion of your opal ring using an ammonia-based product such as Windex or a jewellery cleaning solution (available at all jewellery stores and most retail stores like Walmart or Target). Remember not to soak the ring, as you don't want this solution on the opal. Instead, dip a soft tooth brush into the solution and gently scrub the ring, avoiding the opal as much as possible. Wipe the opal off with a soft cloth as soon as you finish.
Mix a solution of mild dish detergent and warm water in a small basin or bowl. Dip a soft cloth into the solution and wring it out. Gently wipe the opal using the cloth. Dry it thoroughly using a clean, dry, soft cloth.
If your opal has lost its lustre, this can mean that it has microscopic cracks in it. Take it to a jeweller for a professional evaluation and possible sealing.