The Done button will immediately close the dialog and use whatever the parameter values are at that point. It is primarily useful if you have performed a number of single iterations via the 1 Iteration button and want to stop with the last iteration, close the dialog, and generate a report.
The Fit button will iterate until the fitting converges or has other programmatic reason to stop (e.g. reached maximum number of iterations). It then closes the dialog and generates a report.
The OK button is an alternate state for the Fit button. If you click the Fit until converged button or the fit has converged via single iterations (using 1 Iteration button), Fit will change to OK . Clicking OK will close the dialog and generate a report.
Yes, it can be confusing. But I hope this helped.
Thanks, Chris Drozdowski Originlab Technical Support