How to output from OBS to multicast/unicast:
Settings -> Output -> Output Mode -> Advanced
Recording -> Type -> Custom Output (FFmpeg)
FFmpeg Output Type -> Output to URL
File path or URL -> udp:// (working example, tweak it)

Alternatively you can directly send to just one PC in your system by using his IP, for example: udp://
You can use any port that is free and accessible on the PC’s you want to use.

Container Format -> Depends on the protocol, for udp it would be mpegts
Video Bitrate -> 2500 or higher (depends on your encoder)
Audio Bitrate -> 128 or higher (depends on your encoder)

Activate the checkbox «Show all codecs»
Be aware, some combinations and codecs will not work for streaming! You will have to test to find your best combination.
Video Encoder -> libx264 / h264_nvenc (if you have an nvidia gpu) or other codecs can be used
Audio Encoder -> AAC, AC3 or even OPUS can be selected.

Start a Recording, not a Stream.
You can now watch the stream in VLC (for example) on another PC in your local network. Open a network stream in VLC and enter the following address: udp://@ you used this address as the «File Path» earlier, in the case of streaming to one pc you can enter udp://@:9999 on the receiving PC)

ffmpeg UDP in/out syntax
more relay and ffmpeg examples
OBS scene scheduling/time-based switching

If you need to send multicast to specific windows interface:
route add inter.fa.ce.ip