Note that the @ in the filename is correct and that the file should be placed at /etc/systemd/system/onerng@.service - this is due to the file being a templated file that will be called with the device name as parameter.

onerng@.service

[Unit]
Description=Setup OneRNG on /dev/onerng%i
 
[Service]
Type=oneshot
User=root
ExecStart=/bin/sh -c "/bin/stty -F /dev/onerng%i raw -echo; /bin/echo cmd0 >/dev/onerng%i; /bin/echo cmdO >/dev/onerng%i"
RemainAfterExit=true
 
[Install]
WantedBy=sysinit.target