How do I get my coworker to stop barging in on my and everyone else's conversations?

This person is part of your team, right? I wouldn't recommend being rude. It sounds like they are just trying to be a team player, and may have gotten feedback from their boss that they need to fit in better. If they're interrupting work, a "thanks, but we've got it covered" should work, with a conversation to clarify scope if it persists. If they're trying to be part of office social times and it's not interfering with their/your work, I think you've got to politely tolerate it. It sounds like they're trying to make friends. Forming a clique at work that excludes a close coworker isn't nice or professional. If you can find a time to have a private conversation about specific office norms they're violating, that could work. Like, "I know different offices have different cultures, but here we don't visit office parties in the department across the hall."

Oh jeez. I have totally been this person - I can be kind of socially awkward and am always worrying that I'm being "left out" of something. In my last job almost everyone else in the office was part of one team, except for me and maybe 2 other people. I was sharing a work area with a big group of people who were all on the team except me, and I realized eventually that I was butting into their conversations all the time just out of feeling sort of lonely and excluded. It finally hit me that this is what I was doing when one of the women on the team replied to one of my interruptions with "The whole office does not need to be working on this. Just Bob and me." I got the message. In situations where it's not work-related (someone is telling a story, a joke, etc.) there's probably less you can do. Does the butter-inner get left out of the fun/social stuff in the office if s/he doesn't actively intrude? Is there a way you could make him/her feel more included from time to time so that annoying interruptions become less "necessary"?

Maybe you could turn to them sometime and ask for their input.

A friend of mine gently puts people off when they offer uninvited advice by calmly saying, "Thank you, I'll take it under advisement," and then turning back to what he was doing. Try that -- grind to a halt when the co-worker butts in, let them have their say, then simply say "Thanks for your thoughts," and then turn back to who you were talking to without further acknowledgement. They've had their say, you've told them you'll think about it, done.

There are two ways to approach this that I can think of: 1. The curt way. Every time he pulls this, say "excuse me, Bob and I were having a conversation." 2. The compassionate way. The guy may be very awkward socially and not be enclued as to how to interact gracefully. Take him out to lunch and explain that he needs to back off.

Maybe you can try to ignore whatever comment they want to add; basically pretend that you didn't hear them and continue focusing on the conversation with the person you are trying to have a conversation with. Or is there a private meeting area/room that you can retreat to when you know you have to focus on discussion without interruption. I think it's the huge downfall of such a "collaborative" environment (what my company calls it). There have been a lot of distractions and loud talkers in general at times. So much so that I've considered talking to HR to see if they could put together a "collaborative environment rules and etiquette" sort of guide to distribute to everyone.

thanks, we've got it covered.

Have you tried 'this is a private conversation' or 'its ok name, we don't need any help'? Personally I'd just ignore the person and act like I couldn't hear them when they're interrupting but treat them normally at all other times.

Has the chattiness been impacting this co-worker's work output? If so, you could approach it from a performance perspective. Say something like, "Hey, I understand that you really enjoy the social atmosphere here, but we are concerned that it has been impacting your performance. Do you think you could tone it down a little?"