outputInterval and fixedStepSize do not exist in OpenModelica
numberOfIntervals changes the step size (why it's not called stepSize, I do not know). anyway, step size ~ (stopTime-startTime)/numberOfIntervals
tolerance is used in dassl (it knows how large an error there is from integrating a derivative); I'm unsure if it's used when solving non-linear systems or the like
method = "euler" "rungekutta" "dassl" "dassl2" "inline-euler" or "inline-rungekutta" (default dassl). Newer omc versions give an error if it finds that you select an unsupported solver.

Thank you very much for reply.
I'm working in interactive mode..In this case numberOfIntervals, according to the Guide, sets the number of outputs ( and results given ? ) per second..Is that correct?

Thanks,
Francesco