No just that Oz, being a human, had a soul before he was bitten/scratched/whatever, and there is no reason to assume he lost it. Plus his behavior did not change except during the full moon. So not Merle equivalent. Just not without a soul.
From what I understand, all demons were without souls, just some of them did not have to be evil. And no the soul thing never made sense, in part because how having a soul or not interacted with eviltude changed. When did we first see a non-evil demon (other than Angel who explicitly had a soul added)? As I remember, and my memory is not reliable, we did not see one during the first season.
But the Pylea episode showed that the particular kind of Demon who possessed dead human bodies to create vampires was not only a predator, but an animal predator (the "blood beast"). So basically vampires were dead bodies possessed by an evil animal spirit - why they retained so much of humanity (the intelligence came from the human brain, not the demon) but were pretty much over the top evil (the demon part was pretty much purely present oriented and not very rational.)
Other demons were evil or not depending. I guess the ones who could live on big gulps had no particular reason to be evil. So what the soul did exactly I'm not sure. Added guilt? Demons who chose not to be evil seemed not be guilt motivated. In the case of vampires, even those who wanted to stop drinking human blood needed a soul to have a chance to overcome their desire. Also, even Spike got a case of the guilts for a while once he got a soul. Drove him mad for a while.
Even though he appeared to get over it, there are hints he still felt some guilt (some of his conversations with Angel) Just unlike Angel, once he recovered his mental footing, he did not brood and dwell on what could not be changed.
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