According to How do comment replies work? and explicitly stated in What responses will trigger an OP’s red-envelope icon?, you should use @OP's-user-name to notify the OP of a question when commenting answers. On the other hand, I just saw someone (who uses Stack Overflow for some time) commenting answers with @OP.
Does this also notify the OP or is this just a mistake of the commenter?
The question of whether it works has been answered. As for whether it should work — in case someone wants to make this a feature request — I'll say the following:
You could make "@OP" a special case of a less-than-three-char reply tag, but I believe the effort to implement and confusion to future users outweighs the benefit.
There are no shorthand notifications to alert a specific person with comment replies. The only thing that will work is using their name. @OP wouldn't work because it is only 2 letters anyway.
A lot of people do things like @closers, @downvoters, and similar such. They don't do any mechanical effect and are kind of an artifact of how to address people. Many times people also use @name to refer to people that aren't even in the current discussion. I see @Jeff very often for Jeff Atwood in questions he hasn't participated in. It's basically used by some people to identify who is being spoken to/about, rather than to highlight.
Calling it a mistake is a bit of a misnomer - it's probably a force of habit at that point. It's similar to how some people use @ to refer to the poster, when they get automatically notified anyway.
Also, @name should not, to my understanding, work on the OP if the OP has not actually participated in a particular sequence. mmyers clarifies this in the comments.
