Hi,
The decanter looks based on an Orrefors pre-WWII style. However, decanters in this style usually had a foot and a lip, and always at least one of the two. The signature doesn't doesn't looks terribly convincing, and also isn't consistent with how Orrefors applied their codes. "A," or more usually "OA," for Olle Alberius would come before the numbers. The letter after the number usually indicated engraver, and I can't see any engraving on here.
Is the stopper ground or is there a plastic sleeve?
If I had to give an opinion, I'd say not Orrefors, at least not Orrefors the factory we know. Orrefors is also the name of a place and you can find glass with that name on it that wasn't made in the factory.
I hope this helps a little.
David