Convert string to boolean with Pentaho Kettle
You change the type with the Select values step, on the Meta-data panel.
And it works !
However, as the string representation of boolean in PDI is Y/N as well as true/false depending on the context, the guilty step may be the Mongo DB Output.
Right-click on the Oracle Table Input, select output field and check the type of your field. if it is not boolean, convert it with the Select value.
Then right-click on the Mongo DB Output, select input field and check the type of your field. If it is boolean, then it gets silently converted to Y/N. In that case, use a Replace in string step and convert Y/N to "true"/"false" as strings and let Mongo DB silently convert these special string to boolean.