When I mentioned it I knew I really should have looked for a citation first, it's something I heard on BBC Radio 4 (!), which is usually a good source for science/mathematics based information ... but you got me. The article I heard was primarily on empathetic aspects of yawning.
Initially I was going to write "an O2 boost to the brain" but thinking about it I couldn't readily create a mechanism for that to happen - I'm terrible at human biology mind you - and so I reduced the claim in case I'd miss heard. Presumably to boost oxygen to the brain you'd need to increase blood flow (vasodilation or heart-rate) or increase oxygenation of the blood.
Taking a very quick look at scholar.google.com there are citations supporting humans yawning when they have low blood oxygen; there's also papers saying that hypoxic patients increase blood oxygen levels through deep breathing. A yawn is like a single deep breath ... so perhaps?? But I'm not in a position to look further.
