I believe the difference is that Open vSwtich is intended to be used as a software switch similar to how a VM is intended to be a virtual server (and its intended use there would presumably be networking between VMs). While OpenSwitch is intended to be an OS run on an actual physical switch. There seem to be overlaps though since it looks like Open vSwitch will also run as an OS on a physical switch.

OpenVSwitch is a component in OpenSwitch. E.g. OpenSwitch uses OVSDB as its pub/sub configuration store, and also for other state. Other free software / open-source network projects like Quagga have been modified to take their config from that, and store their state to it. Tools like the CLI, the WebUI and REST API then can build on that OVSDB store.

Essentially, one of the aims of OpenSwitch is to build a cohesive API around a wide set of Linux networking tools and daemons, and build a broad community to make it happen.

openvswitch is simply an interface which provides the ability to programmatically configure your network devices.

You could run it on OpenSwitch as the program/module which talks with a controller.