When a single key on a Magic Keyboard starts acting sticky or unresponsive, it’s usually something mechanical rather than a deeper hardware failure. Since it’s only happening with the “o” key and it’s intermittent, the first thing I’d do is check for any tiny debris under the cap. Even a small crumb or bit of dust can cause inconsistent travel. A quick blast of compressed air at a shallow angle around the key often clears it up without needing to remove anything.
Alternately, if that doesn’t help, popping the keycap off is the next step—but do it gently, and only if you’re comfortable. Magic Keyboard keycaps come off with light prying from the top edge, and the scissor mechanism underneath is delicate, so go slowly. Once the cap is off, blow out the area, make sure the scissor assembly moves freely, and reseat the keycap by pressing evenly until it clicks.
If the key still behaves oddly, try pairing the keyboard with another device to rule out a software hiccup. And if all else fails, the scissor mechanism itself may be worn, in which case it may be possible for an Apple Store to swap the key or replace the board. You can contact either your local Apple Store or Apple Authorized Service Provider beforehand to verify this before you go.