Usually the oil only heats the top half of the cylinder, leaving about half the cylinder at the bottom for the solar to work on, especially in the winter, which (hopefully) is the only time you are using your oil heating in the morning.
If you want to use oil in the morning, and leave a full cylinder for the solar to heat during the day, you will need controls.
Most recent solar installations have a controller with a digital display. If not, you can retrofit one, but this is easier if the probes are the same type (PT1000 are the norm, but there are others). You can check the resistance on the probe coming into the controller (eh, disconnect everything first...) - if the resistance is in or around 1,000 ohms at 0 C, then it is probably a PT 1000. It is very handy with a well insulated cylinder to know the temperature top and bottom. Otherwise, you may suddenly find yourself in a cold shower with a mountain of suds to wash off... yech!