The Odyssey² is a console marketed by Magnavox in the US, originally known in Europe as the Philips Videopac G7000 in 1978. It was made by Philips and has a keyboard built into the console.
There were plans to have a Magnavox Odyssey³; however, it was cancelled but ultimately released in Europe as the Videopac+ G7400. It was upgraded with a faster processor, more memory and ran at a higher resolution.
O2EM is the first and the only standalone Magnavox Odyssey² emulator that also emulates the Videopac+ G7400 as well. There is an updated version of the emulator called O2EM2 that works on 64 bit processors since the original version does not support 64 bit processors. Both versions are commandline only.
The emulator does not seem to work on Windows 10.
Odyssey² has been in MAME for a while and emulates all versions of the console including the unreleased Odyssey 3 but is not great with known missing features such as NTSC/PAL switch. Some games are glitchy, some come up with a grey screen and the Chess game does not work because it isn't emulated. MAME also emulates an accessory called The Voice where it adds speech synthesis to some games.
To run it in MAME, you need the BIOS files for them to work. They are grouped with the ROMS rather than the BIOS devices confusingly enough.