Is it legal to create fake online social media profiles with not my real name and post negative reviews of their business?

Let me start by asking you the question of whether you think it should be legal. In fact, it is probably illegal in most states and it is certainly a violation of the Terms of Service of whatever website you will be posting on.

This is not a good idea. You could be creating liability for yourself in many ways.

This is not advisable. You could be held liable from the social media provider for providing false information. And, the business may seek liability for tortious conduct.

Bar is not required to serve you. Involving yourself in a lawsuit when you are facing criminal charges would be an "ungood" move. The smart move would be to not post a negative review and not hide behind some fake ID like a sneak--why don't you simply stand up and use your real name? If you are afraid to do that, I suggest you are not mature enough to be posting anything about a business online.

I am not licensed in Oregon- and so I can't give you Oregon law legal advice. But generally, it is not a good thing to create a fake or false anything. If you want to make a post - make true statement. You can give your opinion so long as it is truly an opinion in many jurisdictions. But the opinion should not be given maliciously or again falsely. Old rule- simply say the truth and own it. I would also suggest that any many states businesses, unless dealing with a "protected class", can refuse service. Run this by an Oregon lawyer. But again be careful what you say. Good luck.