Dear ,

Due to the extreme volatility some market analysts foresee could result in the coming days, OANDA fxTrade will not accept any trading activity from 6:00 AM EST until approximately 3:00 PM EST, on Sunday, June 17, 2012. OANDA believes the convergence of a major market event during off-market hours represents a potential trading risk and has taken this rare step to protect traders from excessive rate fluctuations.
Their website:
Reasons behind our Sunday trading halt

Also this article:
Oanda said halting forex trade during Greek vote