OTOH dates back to the 90s and has since remained very common in internet writing. It is more surprising that you've never seen it than that someone used it.
It also isn't an exact synonym of "conversely".
It also isn't an exact synonym of "conversely".
I've been an extensive internet user for decades and I don't have it in memory, so I'm not sure how to feel about your assertion. I'm not the only person saying this.
I'm sure that depends on the tolerance. "assist" and "help"? "dog" and "canine"? "purchase" and "buy"?