Can it be used for calculations of a circuit on a breadboard connected to the microprocessor?
Yes, Ohm's law can be used for the linear devices, typically resistors, that are connected to the microprocessor. Let's say you want to drive a red LED from a microprocessor pin using a series resistor to limit the current. A typical red LED might require about 10 mA and at that current have a voltage drop of around 2 volts. This means that your series resistor will have 3 volts across it (the 5 volt output from the microprocessor pin less the 2 volts across the LED), and 10 mA through it (the same as the LED). In order for these conditions to exist you should use a series resistor of 300 ohms (3 volts divided by 10 mA).
Note that the LED is a non linear device so you can't use Ohm's law on it and say that it has a resistance of 200 ohms (2 volts divided by 10 mA) since its calculated "resistance" will be different with currents other than 10 mA.
Don