Over the past few days a couple different case manufacturers have started listing Oasis cases on Amazon.com. None of the cases will be available before May, which is good news for consumers because it is a sign that the cases should fit the Oasis correctly.
The cases come from two manufacturers, PopSky and Fintie, and range from a "drop protection" case to "half" clamshells similar to the official Kindle Oasis case (which does not protect the entire rear of the Oasis). The cases come in a variety of colors and are made from either vinyl or pleather, a detail which should please the vegan protesters.
There are no real photos among the product images so we don't know how well the cases will fit, but one thing I can tell you is that none of the 3rd-party cases have a battery, which is to be expected given the $10 to $15 price tags.
On the downside this means that you're limited to only about 6 hours of reading before you have to charge it again, but it also means that you can shave a few more grams off the weight of the Oasis.
I don't think the trade-off is worth it, although I would be interested in seeing how the different cases change the heft of the Oasis (for example, whether the center of gravity changes).