ohm's will not matter as to the draw of the coil. ohm ratings are just the electrical resistance that the wire in the coil produces. These can be calculated into watts and amps also. When i first started vaping i watched MANY youtube reviews and tutorials. i highly recommend someone by the name of grimmgreen he is spot on!
a simple explanation, ohms are the resistance to electrical flow ie. the higher the ohm the higher the voltage you need to get the same wattage. Think of it as a garden hose, volts are the overall amount of water, amps is the water pressure, and ohms are the hose itself. The higher the ohms ie. the smaller the garden hose, the more amps(pressure) you will need to get the volts/watts(water) to the juice.
so if you have a 2.5ohm head at 3.7v that is a decent vape, but if you put a 1.8ohm head on it at 3.7 volt the juice will be much hotter and maybe have a fuller vape. because more electricity can get to the coil to heat it. rule of thumb lower ohm means more vaping power! But be careful as people tend to burn out their ego batteries doing this. if you put a low ohm coil on an ego be careful, do not chain vape it and overheat the batteries circit boards, because lower ohms draws higher amps from the batt.