If your customer demands OSPF as PE-CE routing protocol then you require the folloing points need to be clarified from customer.

1) Area 0
If client is using area 0 then they need to extend area 0 up to SP PE. Authentication can be used as per client requirement.

2) Backdoor Link
If client is using backdoor link in that case SP link will always work as secondary. But if customer demands then SP link can be made primary by creating sham link.

3) Default Route At CPE End
If client demands for the default route at the CPE end then need to clarify one thing whether they are giving a default route from their HO to SP. If yes then SP can forward that link to the CPE by restricting the other routes.

4) Cost in OSPF
Client can use any cost in OSPF, SP will transparently forward that.

5) Multihoming Scenario
If customer is using two service providers out of which one is SP; In this SP cannot provide the automatic failover between links.

6) Dual link with SP
If customer wants redundancy from SP and need to terminate two links from two different SP’s POP, In this case client router will install two routes for outgoing but In SP cloud only one route will be preferred because RR will forward the advertisement to another PE with one best as next-hop. It means in SP cloud no load balancing will happen for client. So if client is using voice type of traffic then there might problem can occur. The best workaround is play with OSPF cost and make single link primary and another will work as secondary.