Here's why OH is still competitive when the national trend has so clearly broken for Obama:
1) Black voter turnout in 2004 in OH was high (10%, 3% increase over 2000) Obama is seeing a large bounce from black voters in some of the other battleground states that is simply not realistic in OH given the already high turnout. Say most likely there is an additional 1% turnout increase vs. the 3-5% you will see in some other battlegrounds.
2) More higher income voters. ($50k+) Bush won them 58/42 in 04.
3) The Republican Get-Out-The-Vote effort in 04 was quite good. 15% first time voters, 46% went to Bush, 54% to Kerry. That's still a big advantage for Kerry but it's probably going to be double that in 08 for Obama.
4) Strong support in southern OH.
So OH is a competitive state. It will really depend on a surge in Democratic turnout along with smaller increases across the board. For example, there are more Latino voters in OH today than in 2004. Obama is doing better than Kerry among Liberals. (14% of them voted for Bush in 04, believe it or not)
Another interesting factor is the state of the race in southern OH where McCain has very strong support. A recent poll showed Obama doing 2% better in KY than Kerry did. That's important because southern OH's demographic is very close to KY. If Obama grabs even 2% of that vote he would hurt McCain's chances at carrying the state.
The most important thing here is the Obama campaign never focused on OH as a must-win. Their strategy from day one has been to win without OH & FL but to stay competitive enough in those states to keep McCain's resources busy. So far, the strategy has worked perfectly.