Well let's see... Natural defenses to the rescue. For lunch, you could feast on beans, cauliflower, broccoli and cabbage then sit next to him at dinner and let it rip which will likely give you a table all to yourself and guarantee he will never sit with you again but then these are you co-workers and you will see them everyday for the foreseeable future so maybe that isn't the best choice.

Form Alliances. Find out how many place settings will be at the table and then make a pact with that many people to join you for dinner effectively putting up the neon no-vacancy sign for your table. If you don't all sit down at once then you can spread your belongings amongst several chairs to claim them or just politely mention those seats are already taken.

You could be direct and tell him to shut his damn pie hole when he eats so that you don't have front row seats for the Discovery Channel HD exclusive on the primary stage of his digestive process. Than again, you have to work with this open-mouthed masticator so you may want to keep the peace.

Use climate to your advantage. If he sits at your table, you could pretend to feel cold, hot or otherwise uncomfortable at your current table and politely excuse yourself to an open spot at another table. Make sure your attire compliments your excuse and plan your table selection around HVAC fixtures so that you can use their presence to your advantage.

Use geography/geometry to your advantage. If the table is large and not round you may be able to have a few pre-arranged co-workers/fiends sit around you and in front of you to block his hideousness from your view. Of course that presumes you have co-workers/friends that more tolerant of his barnyard mannerisms than you. If his reputation precedes him that much your cohorts may mutiny.

In the public interest, prior to the dinner you could leave an anonymous printed note on his desk stating that a number of people feel his table-side ruminations aren't appreciated and that if his mouth at dinner didn't look like the garbage trommel in that episode of Mike Rowe's Dirty Jobs series he might have more friends. OK a little over the top but it leaves little to the imagination and little doubt he gets the point.

Lastly, feign concern for his social well-being; given the rare chance that he his concerned with his personal appearance you could point out something "caught" in his teeth and after he attempts to remove said fictitious food scrap you tell him, "Nope, I still see it." Continue with this game until you finally say "If you keep your mouth closed most of the time people probably won't notice it." He might be a little apprehensive about opening his gaping maw if he knows everyone will be looking at the spec on his teeth. Then again, he doesn't seem to care about showing everyone a mouthful so what difference would 1 spec make?

I'm sure there are other ideas and hopefully these will spur some of your own. Have fun with this and let us know how it went.
Sleepyhollow

Sit and watch several episodes of Little House On the Prairie. Fast forward to mealtimes with Pa. Eat chocolate at the same time. You'll learn to enjoy the open mouth eating, ie. pavlov's dog.

Good luck - humans are disgusting creatures.

I see we are amused, huh? My workgroup had a habit of going out to lunch on group member birthdays. That"s how I discovered his habit. I was a new hire, they had been doing this for years. Our manager would tell him to close his mouth, but alas, that habit is too ingrained.

I started declining the birthday lunches. When the manager retired, the lunches stopped altogether. I think people went only because the manager enjoyed them so much.

Thank god for Moms. She suggested I "out mingle" him at the dinner, watch where he puts his jacket or makes his seat selection, then sit at a different table.

I had a co-worker with bad breath once. I tolerated it until she told me she was on her way to talk with a bigwig client. I said nothing, just reached in my desk and handed her a mint. She got the message.

I had a coworker with underam odor. Pretty girl. Never knew what to say to her.....

This open mouth chewer is just a crass guy. Loud talker, loudly crinkles potato chip bags at group meetings. If the office is quiet, he has to play his radio loud enough to be heard two offices down. I know, because I sit two offices away from him.

I just need a plan......if he asks me where I am sitting...? I would love to say "any where you are not because you chew with your mouth open and I cannot stand that".

Sounds to me like this is a big enough problem that you need to talk to him about it. Remember that this is a problem for you, not for him, unless he decides it is a problem for him that you are bothered by it.

I joked earlier, but really I understand. Nothing I hate worse than mouth noises of any sort - but I recognize that as my own issue. People have to eat, and honestly some people were born in barns. So, when I'm bothered, I distract myself with other noises or I get up and walk away. And, yes, there have been times when I've said something, but seldom does that change anything. You're better off changing your own situation than trying to change someone else's behavior -

I wouldn't talk to him about it. He is certainly wrong to chew with his mouth open, of course, but it is a big no-no to correct other people, even if you don't do it in a nasty or humiliating way. (And as others have pointed out, it doesn't work, anyway -- and you say the manager has already tried.)

If you were his best friend, maybe you would consider whether to discuss it gently with him. But you aren't. And it's not like he is making bigoted or nasty remarks or stealing -- eating with his mouth open is rude and gross, but it isn't mean, illegal, or unethical.

I'd just let him choose a seat first -- go to the rest room if you need to avoid his asking you where you are sitting -- and then choose one far away, or at least on the same side of the table, so you don't have to look at him.

And if all else fails -- just don't look!

if he asks me where I am sitting...?

I would just say "Oh! I'm not sure yet. Don't worry. Go ahead." Then sit away from him and don't look his way.

I had a coworker with underam odor. Pretty girl. Never knew what to say to her

A friend of mine had this problem. She'd smell underarm 15 minutes after taking a shower!

The interesting thing about body order, those who have it smell themselves and assume that those around them smell the same way. This is not a joke. Some years ago I was in a civic choir which had a gentleman recently from England. He evidently was not accustomed to wearing deordrant. After several uncomfortable sessions we politly took him aside and explaned the situation. We was grateful and the problem was solved. The man who eats with his mouth open probably comes from a home where every one did the same so he thinks it is normal.

On the other hand, perhaps he came from a family of confirmed jerks!

To catch you guys up:
The celebration started at 4PM, ran to 8PM. We all started leaving the office about 4:05 to drive to the hall. Just as I was getting my coat, my phone rang. It was a long-winded client...I was saved! I told the others I'd meet them there. I arrived about an hour later and sat at the late people table..which was great! And, our table won the mystery contest that was being held.

Funny...things just work out right sometimes.