Comments by "John Burns" (@johnburns4017) on "A Complete Guide to Titanic's Engines" video.

SS Olympic was converted to oil in 1919. The coal bunkers were converted to oil tanks. Large burners were fitted into the boilers. Efficiency was not a great as with boilers specifically designed for oil burning, but far better than coal burning. By the late 1920s very few coal fired steam ships were about. In the 1930s ships were moving quickly to diesel engines in new designs and refits. However the British designed Liberty ships of WW2 were specifically designed with now old technology, reliable, piston steam engines that could run on coal or oil boilers, in case oil was not available. So many Liberty ships were built that there were more steam ships in 1945 than in 1939.
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A problem was that the centre turbine powered propeller could not reverse. As the two outer props were put in reverse the centre prop was acting against them. I believe there may have been a way of shutting it down (not 100% sure on that in Olympic class).
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