What do people mean when they say OG Gamertag?
An original gamer-tag is a one worded gamer-tag that is a daily/commonly used word which has no prefixes or suffixes. Examples are ("Dog", "Cat", "Doctor", "Theft", and millions more.)
A semi OG gamer-tag has either a prefix or a suffix. Examples are ("Dogs", "Cats", "Doctors", "Thefts")
Having a random word from the dictionary such as "tesseratomy" is not OG, not semi. Just because it's in the dictionary doesn't mean it's good.
What's the history behind the account?
Before purchasing a OG gamer-tag or a semi OG gamer-tag make sure to ask for history on the account, ask the seller if he has been the only owner of the account, how long has he had it for. Any warnings/ever been banned? How many consoles are on it? Doing this will give you a better understanding of the account.
How to make sure the gamertag exists
Send a message to the account and ask him to re-read the message back to you.
Ask him to send you a message from the account.
Here are some more things you can try;
Go to www.xboxgamertag.com and search the tag you're buying
Check if the bio/name are "Code of conduct" (meaning it's either banned, defaced or had the bio changed from mobile version)
If it has the original xbox avatar (white background & green X) or no avatar - this means it's a v1 gamertag and DOES NOT exist on the 360 platform.
Check when it was last seen online if it was more than 6 months ago - ensure you follow the message guideline above. (credit to morbid)
What to do when swapping a gamertag
If you're selling a gamer-tag please don't tell anyone about you swapping it to a fresh account, matter of fact don't even put the tag up for sale without it being on a fresh account. I have fallen victim to this before and it's a sucky feeling so here's what to do. DONT tell anyone you're swapping it, wait several weeks or months until you swap it. If you tell people it will get turboe'd meaning as soon as you swap your tag it will get automatically took. If the buyer is asking for it to be on a fresh they are most likely wanting to turbo/take your tag.