OneLang has a hard time defining itself.
Some may call it a transpiler, a special compiler which can compile source code from an input language to another. Those will have a hard time using it, as OneLang has its own rules and does not respect those of the input language, sometimes not even its syntax.
So the question arises, is OneLang a new programming language? Although we can talk about it with words like object-oriented or generic or strongly-typed, it has its own type system and AST, would you call something a programming language which does not have its own syntax?
Thus let's just define OneLang as the following for the time being: OneLang is a tool which helps writing code in multiple langauges at the same time.
It doesn't solve the problem for you, it just helps you a bit in it. You still need to master the target languages and OneLang to be able to produce anything usable.
Comparison: https://github.com/onelang/OneLang/wiki/OneLang-vs.-Haxe-vs.-Progsbase-comparison