The OS-8 is a series of oscilloscopes built for the U.S. Navy/U.S. Army, I'm not sure exactly which manufacturer yours is from, but an "Instruction Book" is here, which includes instructions for "Corrective Maintenance" as well as both a pictorial wiring diagram and schematic wiring diagram, which will prove invaluable if you need to service it (and are also quite pretty, see pages 46-47). It appears the the OS-8C/U and the OS-8E/U are very similar, so manuals for one are sufficient for the other.
See also TM11-1214A, the technical manual for the OS-8C/U, which is very similar to the document above, but not identical (perhaps Army vs. Navy). It seems that this oscilloscope was manufacturered by Carol Electronics Corp. (Martinsville, WV) but it could also be from The Hickok Electrical Instrument Co. (Cleveland, OH).
You may find the time base behaviour to be a little strange, because this scope uses a "repetitive time" horizontal sweep, rather than a triggered sweep, which is what most modern oscilloscopes use or emulate. In other words, the sweep is driven directly by a sawtooth generator, and the sweep frequency must match (or be an integer multiple) of the input signal to get a stable display, and therefore it's rather difficult to measure the actual time shown by the scope.