Both "oauth-" and "ob-" providers operate using the same principle, where the user is redirected to their bank's Internet Banking page. The end-user authenticates for a Third Party Provider (TPP), like TrueLayer, to gain access to listed endpoints, e.g. /accounts, /balance, /transactions etc., and the end-user is directed back to the TPP to carry out their function. Using this method, the end-user does not need to share their Internet Banking details with the TPP, which is part of the enforcement of Open Banking.
Open Banking has a set of regulations that major banks need to follow, known as PSD2 regulations. You can find more information on their official website.
All major banks that are required to adhere to PSD2 regulations will have an "ob-" connector. You can find a list of supported banks with "ob-" here.
Challenger Banks, for example, that are not required to follow PSD2 regulations, generally adopt for "oauth-" connectors, though they are not limited from using "ob-" connectors. You can find a list of supported banks with "oauth-" here.
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