Have a look at the IPv6 HowTo on the OpenWRT wiki. It's a pretty good starting point. I'm not going to give a detailed guide here, but a point-form summary of the broad steps to take.
First, choose a static IPv6 address from the /64 block your ISP gave you, and assign that address to the LAN side of your OpenWrt router.
Next, install and configure
radvd in OpenWrt, to advertise itself as the default route and DNS resolver to LAN clients (clients should be configured to use IPv6 Stateless Autoconfiguration, unless you prefer to use static IPs, or DHCPv6).
And finally, configure firewall rules (remember that there is no NAT in IPv6, so it is essential to have a stateful firewall to prevent outside hosts from initiating unwanted connections to hosts inside your LAN).