The spelling 'czar' was almost universally used in the U.S. well into my adulthood. I am now in my 80's. During WWII we had various people in the government running imnportant programs who were dubbed 'czar' of their program, whether it had to do with manpower or some industry or price controls. The word morphed into 'tsar', which my limited knowledge of Slavic languages would suggest is closer to the Russian, sometime later, probably gradually through the '50's to 70's--much more recently than a century ago. I can recall noting the change and wondering why. If the OED doesn't cite usage to indicate when the change took place, I don't know who would.