Meaning of “Onechan(ワンチャン)”

“Onechan(ワンチャン)” means “Low possibility” or “There might be a possibility” kind of…
This slang is from “one chance”.
This slang has been around for quite some time and has become fairly established so far, I think.

We use like this
“Onechan arussho(ワンチャンあるっしょ)” which means “There might be a possibility”.
“Onechan ikerukamo(ワンチャン行けるかも)！” which means “It might be possible to go, perhaps”

Example Conversation

A: You like her?
B: Well, yeah, but she’s popular, so there’s not much I can do.
A: Hmm, I think there is OneChan(ワンチャン)

A: 彼女のこと好きなんでしょ？
B: いやー、そうだけど、彼女モテるし、どうにもなんないよねー。
A: えー、ワンチャンあると思うけどなぁ。