Users with European Oanda accounts cannot trade through them on TradingView. Here's the message from Oanda:
On 16 December 2017 Oanda is required to restrict API trade execution from 3rd party platforms for European clients (Motivewave, TradingView, Seer, MultiCharts, etc.). You may still use a 3rd party platform for trading analysis, however, you will no longer be able to execute trades on 3rd party platforms that execute trades via an API.
Your OANDA account enables you to execute trades through OANDA proprietary platforms or MT4. This change is required to enable OANDA to meet the anticipated regulatory requirements at the beginning of 2018, and consequently for you to be able to continue to trade with OANDA.
The Oanda Web Platform, however, should be quite familiar to TradingView users.