So basically, if the author is not responding, you can't react on it?
[opinion]Reaction videos are crap and you should work to create sweet, high quality content.
If it's a reaction video formatted the way that many other reactors are formatting their stuff and you're not creating original content, personally I believe that you shouldn't even be posting it.
Since you're literally ripping and re-uploading someone else's content in it's entirety, which is against YouTube's community guidelines. (and for some reason they aren't cracking down on huge reactors) [opinion/]
In addition, part of the reason it's bad, is because you're literally stealing views from the OU's channel and using it to create derivative content, without adding any value.
Now, if it's like 5 seconds in length and the majority of the video is the content that YOUR channel has made, then sure; it should be fine.
P.S. I don't mean to bombard you with a wall of text but I hate content thieves and this is somewhat a knee-jerk reaction to the subject, but it really grinds my gonads. Also, react channels don't add significant enough value to be able to call the content that they put out, their own;
in my opinion.
P.P.S I think gradeAunderA put out a very informative video on the subject and he fairly clearly and concisely outlines the issues with reaction videos. (I'm unsure as to whether or not I can link it here so I won't)
tl;dr If it's quick and a little snippet of video, it's probably fine. If it's anything longer than like 10 or 15 seconds or over 40% of the original video, then you probably shouldn't. Unless you have actual WRITTEN permission from the copyright owner, then do whatever your agreement says.
P.P.P.S As I said earlier, you should work towards, sweet-as-hell awesome content that you already know you can put out! Reaction videos, imho is not the right route. Strive to be the best and you'll get there man.