As long as it's a small, non-locking pocket knife, you're fine. If it's larger, you're fine as long as you have a good reason (eg. needed for work).

That's dangerous advice in many areas of the United States. In NYC if a police officer finds any kind of folding knife, regardless of how harmless, if he wants to arrest you he probably can under NYPD's absurd perversion of the law by considering it possession of a drop knife. If the officer can open the knife by holding it and flicking it open via inertia then he can arrest the victim for possession of a drop knife. They usually don't hold the handle to try and flick it open either, they'll hold it from the tip of the blade as that makes it very easy to just arrest anyone with a pocket knife.

https://news.ycombinator.com/item?id=15372404

The first man mentioned in that article was convicted and sentenced to 6 years in prison on the sole charge of possession of this knife.

https://www.amazon.com/Sheffield-12113-One-Hand-Opening-Lock...

Try with a slightly-stuck Opinell. You're more likely to throw the knive or cut/puncture your finger than you are to flick it open. You can push it open with the thumb while retaining the handle with the ring? finger, but that's not flicking.

If the blade is under 3" and isn't a locking blade you don't need a reason. If the blade is longer than 3" you need a reason. "I'm taking this bottle of wine to my friend's house and they don't have a corkscrew" would be fine.