Oneness was the state of union with the Force. While under normal circumstances this could only be achieved after a Force-sensitive had died and passed into the Netherworld of the Force, this was not always the case.
In some circumstances, individuals managed to become akin to a living manifestation of the dark side of the Force—or even the light side of the Force, although they were prone to severe temptations to the dark side. They essentially became nothing more than an extension—a tool—of the Force, and were at a perfectly calm state.
Luke Skywalker was able to naturally achieve oneness with the Force more than once. Barriss Offee was also able to achieve this state two times with the help of the bota with which she injected herself.