If your existing wired network uses IP addresses outside the range of 192.168.1.X, you can simply plug the WAN port into the wired network and the router will have access to the Internet. Leave your PC plugged into a LAN port.
If the IP range does conflict, you would need to change either the OpenWrt LAN IP, or the IP of the existing main router.
Or take another approach of making the OpenWrt router a LAN device. To do that, set its LAN IP to something that is in the range of the main router's network, but not used by any device. Also you need to add a gateway and DNS to the LAN configuration:
config network 'lan'
option type 'bridge'
option device 'eth0.1'
option proto 'static'
option ipaddr '192.168.1.250' #Change this from 192.168.1.1
option netmask '255.255.255.0'
option gateway '192.168.1.1' #These lines point to your main router
option dns '192.168.1.1'After doing this, reboot the router or restart the network and connect the existing network to one of the LAN ports. Then ssh to the router at 192.168.1.250.
Of course the "lan device" approach can be used no matter what the IP of the main router is, you just have to be sure to configure to match it.