REUTERS/Ralph Orlowski
"Outright Monetary Transactions", known as OMT, just mean that the European Central Bank would buy a country's government bonds from investors, if it found itself in financial distress. The mechanism was announced in the summer of 2012, after ECB boss Mario Draghi's "whatever it takes" speech.
OMT has never actually been used, but it is credited by the ECB with slamming the brakes on Europe's sovereign debt crisis. Effectively, the guarantee that the central bank would not allow individual countries to default made their bonds seem much less risky.
It's been the subject of intense debate and scepticism ever since. German politicians took legal action against the move, and the highest German court decided that it was probably all illegal, but agreed to refer it up to the ECJ.
However, the court is insisting on a number of conditions, listed in a link attached to this FT article.