This is more of an interpreter than a compiler, but if you look at the "Arithmetic example with semantics" [1] linked on Ohm's GitHub page, you can see how Ohm can be simultaneously used for both (1) defining a language for arithmetic expressions and (2) defining how those expressions should be evaluated. I don't think Lezer even tries to help with something like this.

[1] https://jsfiddle.net/pdubroy/15k63qae/