DS Server's OAuth implementation allows the "code" and "token" grant types. The recommended grant type is the code workflow that is considered more secure.
In DS Server, you can enable/disable the "Client Credentials" grant type for each security profile. By default, the "Client Credentials" grant type is disabled. In order to enable this grant type, check the Allow "Client Credentials" grant type check toggle button. It is rendered in green, if this grant type is enabled: