What could be making them potentially competitive?
Well, both candidates went to Yale Law School, so that's helping with the fundraising.
Harbaugh is running in a very conservative district, but his military background is serving him really well. Rader, meanwhile, is running in a decently competitive district where the ODP has repeatedly run into recruiting failtures.
could OH-15 be competitive?
Unfortunately, no. Especially now that Stivers is serving as NRCC chairman.
The 15th district has a pvi of + 7 Republican. I'm actually surprised it's that low and question whether that stat from Wikipedia is up to date. Regardless, it is more than gerrymandered enough to keep it safe R even without Stivers running.
Keep in mind, Athens is in the district. That's probably what keeps the PVI at "only" R+7.
