According to oAuth specification, section 11.3,
While OAuth provides a mechanism for verifying the integrity of
requests, it provides no guarantee of request confidentiality. Unless
further precautions are taken, eavesdroppers will have full access to
request content. Service Providers should carefully consider the kinds
of data likely to be sent as part of such requests, and should employ
transport-layer security mechanisms to protect sensitive resources.
It is clear that if your request does not have secure data e.g user_id=2&messageId=33, than there is no need for https, but in 2-legged scenerio, where you are passing password while getting access token, you must have to use https at that time.
In both cases, either 2-legged or 3-legged, the rule is, when you are updating/fetching secure data (e.g. Credit Card updation, payments, passwords), you must have to use https.