I have 3 routers connected in a ring fashion with one (RB450G) connected to the internet and natted. The other are RB750G connected to the 450 on different ports with different subnets, I have ospf sharing routes between them and all IPs are private. The 750G s have hotspot and pppoe clients on other ports.
The question is do I do natting on the 750Gs for for the clients to get internet or ospf will route the traffic and leave the 450G to nat?

There's no need to NAT until you route out to the Internet. Best practice would be to NAT on the 450Gs.

Just as I thought of doing.
Thank you.