Just gonna leave my answer here from movies stack:

Harry never knew he was a Wizard or could use magic in the first place to suppress magic. He just thought it was "strange"

The Dursleys never liked to acknowledged Harry's magical roots. They just wanted to ignore it. Harry was forbidden to do or mention magic but not beaten about it the way Credence was beaten. He was hidden from guests, lied to about with neighbours, changed schools, but not suppressed.

Also most importantly, Harry never held back magic. He made the glass disappear, he ended up on his muggle school ceiling top when Dudley and his gang were chasing him, he ballooned up Aunt Marge. So he had his own outbursts but they we not exactly parasitical like Credence's

An obscurial arises from forcefully suppressing magic. Harry was never suppressed, and was in a lesser horrible condition as an obscurial (eg:Credence) was.

That is probably the reason Harry never was an obscurial.

One example of someone who might have been an Obscurial was Arianna Dumbledore.

"It destroyed her, what they did: She was never right again," Aberforth said. "She wouldn’t use magic, but she couldn’t get rid of it; it turned inward and drove her mad, it exploded out of her when she couldn’t control it, and at times she was strange and dangerous. But mostly she was sweet and scared and harmless."

"[If] the Ministry had known what Ariana had become, she’d have been locked up in St. Mungo’s for good. They’d have seen her as a serious threat to the International Statute of Secrecy, unbalanced like she was, with magic exploding out of her at moments when she couldn’t keep it in any longer."

I can be almost sure Ariana was an Obscurial.

Just gonna leave my answer here from movies stack:

Harry never knew he was a Wizard or could use magic in the first place to suppress magic. He just thought it was "strange", He NEVER tried to suppress his magic once ! (you need to suppress your magic to be an obscurial)

The Dursleys never liked to acknowledged Harry's magical roots. They just wanted to ignore it. Harry was forbidden to do or mention magic but not beaten about it the way Credence was beaten. He was hidden from guests, lied to about with neighbours, changed schools, but not suppressed.

Also most importantly, Harry never held back magic. He made the glass disappear, he ended up on his muggle school ceiling top when Dudley and his gang were chasing him, he ballooned up Aunt Marge. So he had his own outbursts but they we not exactly parasitical like Credence's

An obscurial arises from forcefully suppressing magic. Harry was never suppressed, and was in a lesser horrible condition as an obscurial (eg:Credence) was.

That is probably the reason Harry never was an obscurial.

One example of someone who might have been an Obscurial was Arianna Dumbledore.

"It destroyed her, what they did: She was never right again," Aberforth said. "She wouldn’t use magic, but she couldn’t get rid of it; it turned inward and drove her mad, it exploded out of her when she couldn’t control it, and at times she was strange and dangerous. But mostly she was sweet and scared and harmless."

"[If] the Ministry had known what Ariana had become, she’d have been locked up in St. Mungo’s for good. They’d have seen her as a serious threat to the International Statute of Secrecy, unbalanced like she was, with magic exploding out of her at moments when she couldn’t keep it in any longer."

I can be almost sure Ariana was an Obscurial.