Just because you have millions or billions to spend, does not make the domain name a steal.
Omg.com is definitely not worth 80k in my standards. As chillking has said, 30k, and that is probably tops.
Also, most big companies will NEVER negotiate direct purchases on big domains. They usually use a third-party or a shell company, or in this case - Sedo.
OMG.com really has no end-user value to be honest as a name itself. I'd also put it around 30k. And in the current market, perhaps even less. Sure to Yahoo it is not a lot of money for a domain name they really want. We've witnessed Ask.com buy out a bunch of nearly worthless domains lately for millions. But that doesn't say much about the value of the name. When you have money, you are likely to spend it much easier.
But congrats to both Yahoo and seller.