If i open CPF-IA with DBS, does that mean that i can only buy FD from DBS and cannot buy from OCBC using CPF-IA?

Currently, i am very confused with the different terms used online so would like to have some clarifications. Also, is buying FD from CPF-IA the same as using money directly from CPF-OA?

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I was wondering if lets say buy from DBS in this year, then next year OCBC FD rates are better. How easy can i go about using CPF-OA money to do so? Do i need to change CPF-IS bank to OCBC if i want to use my CPF-OA for FD next year?