Okay, so the O-rings look fine and the device is dry, but it's still weeping at the connection. What's left?

Residue buildup. If you've been using thicker e-liquid, residue can build up around the airflow ring and create a path for liquid to escape. Dip a cotton swab in isopropyl alcohol and wipe the ring and the channel it sits in. Let it dry completely before reassembling.

Check the mouthpiece seal. Some mouthpieces have their own O-ring. If that one is worn, it can leak even when the airflow ring is fine.

Look at how you're storing it. If you're laying the device flat with a full tank, gravity is working against you. Store it upright when it's not in use. That's a simple fix that prevents a lot of leaks.

If you've done all this and it still leaks, the O-ring may be too worn to seal. That's a part that can be replaced on some devices—check the manual.