It does not appear that Facebook is currently working on a virtual reality world similar to the "OASIS" from Ready Player One. While there is evidence that Facebook has been looking into building a VR platform, there is currently no indication that it is working on a VR world similar to that in Ready Player One.
Facebook has been investing heavily in virtual reality and is making tremendous strides with its help. It is actively working on a technology called "Facebook Horizon," which is described as a “shared virtual world” where people can interact and explore. However, it does not appear that this technology will provide an experience similar to the one found in the world of Ready Player One.
That being said, the public is not privy to all of the development projects Facebook is working on. It could very well be that the company is developing an OASIS-like experience, but that has not been publicly known.
Ultimately, it does not appear that Facebook is currently working on a VR world as seen in Ready Player One, but that could change in the future.