the onc you've referred to is actually a reserved server api - you don't have any access to it as a client. there is however the mql language which you can write expert advisors with, and it is supported with any metatrader broker.
there is also an api with oanda - pure java/c (perl also i think?) but it will require you to program everything from the start as it doesn't have any readily available functions, unlike the mql language.