It's not really up to you. There are two basic possible configs that ISP can use. First is routed subnet, where config from ISP's router would look like this:
add interface=<to_customer> address=188.8.131.52/30
add dst-address=184.108.40.206/23 gateway=220.127.116.11
There would be one connecting subnet, in this example 18.104.22.168/30 and you'd have 22.214.171.124 on WAN and use 126.96.36.199 as default gateway. And whole 188.8.131.52/23 would be yours. But what you describe looks like not routed subnet, where ISP's router has:
add interface=<to_customer> address=184.108.40.206/23
If that's the case, ISP expects all addresses from /23 to be directly reachable on same link. And if you need them routed further, you have to use proxy ARP. I'm no OSPF expert, you can probably use it in your network with these addresses, but you still need to have proxy ARP on WAN, otherwise ISP wouldn't be able to communicate with them.