Germany's Federal Court challenged the OMT's legality in the European Constitutional Court of Justice, arguing the ECB was acting beyond its mandate, effectively financing government deficits.
In his opinion, published on Wednesday, Advocate General Villalon argued that the ECB must have a broad discretion when framing and implementing the EU's monetary policy.
He said it would first have to spell out its justification for any bond-buying programme and not be involved in any direct aid programme to any eurozone member state involved.
He also warned that the courts lacked the expertise and experience of the central bank in this area and should be careful about criticising the ECB.
Wolfgang Kuhn, Aberdeen Asset Management's head of pan-European credit, said: "The opinion criticises the German court for daring to refer the case the way it did and seems to say that it can't be up to national courts to judge what goes on at a European level.
"The opinion removes another inconvenient obstacle to [ECB president Mario] Draghi's big quantitative easing gamble, which will now almost certainly be announced next week. But the voices that see both OMT and QE as a blatant breach of EU treaty law will only grow more shrill."
The Advocate General's opinion is an influential legal proposal to the Court of Justice, but is not binding.