How to measure a temperature with a NTC thermistor and an Op Amp?
20°C to 100°C is a fairly wide range for a thermistor with \$\beta=4080\$- you will be dealing with a large dynamic range (about 30:1) or you will have to add some parts to sort-of roughly linearize it. 

If you measure the resistance you can use the Steinhart–Hart equation to calculate the temperature from the resistance. 



The resistance change of a 10K thermistor for 0.1°C at 20°C is about -58 ohms. At 100°C it is more like -1.9 ohms, so the measurement at higher temperatures will tend to be noisy or jumpy (perhaps quantization noise) if the resistance measurement is linear.  

Cheap temperature meters convert the resistance into a relative frequency count (digitally) compared to a reference resistor and use an equation, however the measurement is done typically to a resolution of more than 12 bits. 



The simplest way with an ADC (as in your Arduino) is to use a series resistor to Vdd close the thermistor resistance at the temperature of most interest to get a ratiometric measurement. For example, if your most important temperature is 60°C you might pick something like 2.490K (that will cause some self-heating that has to be evaluated). Then it's straightforward to calculate the resistance from the ADC count, and then use Steinhart-Hart for the temperature (or a simple lookup table is better in this particular case, which can be pre-calculated using Steinhart-Hart). You don't need the op-amp at all.