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.