Something I like to bring up is the idea that Onimaru did not connect Zoro's resemblance to Ushimaru. If we go back to the chapters he first confronted Zoro, his behaviour around what was a total stranger, was off.
All he does is take back Shusui and didn't attempt to steal Zoro's other swords. He later lectures him about Shusui's importance, and later expresses disappointment in Zoro's response to the lecture.
Now think about it, Onimaru had just seen Shusui after over 2 decades missing, and he stumbles upon it in the hands of this stranger. As far as he should be concerned, Zoro was a no good thief galivanting with the country's treasure. Despite that, he didn't attempt to steal Zoro's other swords, just Shusui. He states he "over-estimated" Zoro when he had never met him till that point, suggesting he had expectations for Zoro once he laid eyes on him. He was then seen smiling after Zoro defeated Kamazou, only stealing the latter's weapons. (Though defending innocents probably left a bigger impression on him at that point, but still worth noting.)
Onimaru was by Ushimaru's side for years. If anyone could spot a resemblance between Zoro and his late lord, it'd be him. And based on how Onimaru treated Zoro vs how he treats other ruffians he barely knows, has me leaning on the idea he could see the similarities, and that's what informed him on Zoro's character initially. Even moreso, he's a fox and likely recognized Zoro to be related to Ushimaru based on his scent.