o

inline val <T : Any> Assert<T>.o: Assert<T> (source)

Inline property referring actually to this and allows to write nicer sub-assertions.

For instance, instead of:

assert("hello world"){
  this startsWith "hello"
  this ends with "world"
}

You can write

assert("hello world"){
  o startsWith "hello"
  o ends with "world"
}