Well it's sort of sort of debatable. In his fight against Inaba, Ohma was pretty much dominating even in base with brute strength, it was only when Inaba started using his hair that Ohma was starting to struggle, which makes sense since the Inaba clans hair is noted to far stronger than normal hair. But the second Ohma brought out the Advance he completely stomped Inaba and destroyed his hair with raw power, it's noted several times throughout the fight that Inaba was helpless against Ohma's brute strength.
Against Raian, it's literally stated by serveral prominent fighters that Ohma was surprisingly strong post training. Even the Fang was perplexed by Ohma's sudden boost in strength. Even Yamashita, who's observation skills are top tier, noticed the strength boost. So there's literal proof that his strength did increase. Of course, Ohma still gets stomped by base Raian through brute strength alone, and it's only when using Posessing Spirit that's he's able to do significant damage to Raian, who also acknowledges that Ohma's power had increased since the the first round. And that was just when Ohma was using the Advance, he hadn't even managed to bring out the full power of the Possesing Spirirt, which allowed him to fight on par with 100% Raian. Of course, once Possessing Spirt runs out, Ohma is forced to rely on skill and technique to out Raian down, but you still can't ignore all the previous statements and showing of Ohma's increased strength.
Against Cosmo, it was mostly a match of skill, since both Ohma and Cosmo were too beat up to bring out their full strength. Plus Cosmo isn't really a striker in the first place due to his small frame, and chooses to make up for his lack of strength through his supreme grappling. Plus, it's clear that Ohma was stronger than Cosmo in terms of raw strength since the beginning of the match, since Cosmo took extreme damage from Adam in the first round, who's pretty low tier in comparison to even round one Ohma.
Against Wakatsuki, Ohma was pretty much struggling the whole fight. While it is impressive that he could survive the monstrous attacks from Wakatsuki, both with and without Indestructible (which is more of a durability feat), it was only through the Demonsbane that he was able to since since he could divert the force of Wakatsuki's blows back at him. It was noted by Ohma that Wakatsuki surpassed him in power. Ohma even started that Indestructible was useless against Wakatsuki's strength. However this is also showing of his increase in power, as he went from getting OHKO'd by a casual Fang and gettin stomped by base Raian to being able to survive blows from then
So while he did rely on technique to help him towards the later parts of the manga, it's plain to see that he also saw a boost in strength and durability along the way. He's not the most physically powerful fighter in the series, as shown by his fights against Raian and Wakatsuki, who're pretty much the top tier along with Reinhold in terms of pure strength, but he's just strong enough that, in combination with the Niko style, he can stand toe to toe with these guys.