Connecting both networks through the OpenWrt router is trivial, you just need to add two forward rules (one I'm each direction). The problem, as pointed out by @andyboeh, is how to tell devices on each network that they should use the OpenWrt box to reach the other network.
In OpenWrt, having "2 interface talk to each other", is part of the firewall configuration. Even if you do not want a firewall, it's much easier to follow the advice received, and use the firewall configuration tools.
If this OpenWrt box is not the main router in the LAN network either or both networks, the network devices will not know to route through it to reach the other LAN. The best solution is to install static routes in both main routers to point to the other LAN via the OpenWrt box's IP on the local LAN.
In subnet 1 main router install route 192.168.1.0/24 via 10.0.0.2
In subnet 2 main router install route 10.0.0.0/24 via 192.168.1.12
Again these need to be configured in the network's router, not the OpenWrt box in question here.
These routes could also be installed manually on the network endpoints, or pushed to them by DHCP, but Windows in particular is not built for routing and may fail to handle them properly.
If you set the default firewall forward rule to ACCEPT (the default is REJECT) and make sure not to assign either interface to a firewall zone, the firewall will allow forwarding between such unassigned zones. This is about as close as you can get to having no firewall.