what do 'o' and 'b' indicate here?

Two cowboys, a newcomer and an old-timer, are drinking beer in front of a saloon. Suddenly, there is a clatter of hooves, a great cloud of dust, and something moving extremely fast from one end of town to the other. The newcomer looks at the old-timer, but seeing no reaction, decides to let the matter drop. However, several minutes later, the same cloud of dust, accompanied by the clatter of hooves, rapidly proceeds in the other direction. Not being able to see what’s behind the dust, and unable to contain his curiosity any longer, the newcomer asks: “OK, what the hell was that, Bill?” / “Oh, that’s Uncatchable Joe. Nobody has ever managed to catch him, Harry.” / “Why? Is he so fast, Bill?” / “Nope, it’s just because nobody needs him, Harry.”

Source: https://medium.com/@EwardEd/the-uncatchable-joe-or-a-life-that-you-want-5825068beabc

https://www.youtube.com/watch?v=9LDZu1SwUCc