Hi, I have sent a support request to Oanda and they replied... v1 REST API will soon die.

Is there anybody already using v20? How it works? Broker plug in AAAARRRGGGGHHHHHHH...

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Account number like "XXX-XXX-XXXXXXX-XXX" signifies a v20 account, which you need to use with v20 REST API, you can add this by logging into oanda.com and clicking on add-sub account. v1 REST API is set to be phased out in the coming year so we no longer provide that.
Make sure you use this documentation http://developer.oanda.com/rest-live-v20/introduction/