Parts of the loop() run very fast, for example the part that writes to the display.
Could you add a delay(10); or delay(100); at the end of the loop() ? Then there is a delay to make the Nano voltages steady before doing the next analogRead().
A OLED module runs at 3.3V. It might work at 5V, but its SDA and SCL signals are still 3.3V signals. As far as I know, only Adafruit makes a OLED that is compatible with a 5V Arduino board.
Using the avarage of a few samples is a good way to reduce electronic noise.
long total = 0;
const int n = 10;
for( int i=0; i<n; i++)
{
total += analogRead( A0);
}
total += n / 2; // add half-a-bit correction
VRT = float( total) / float( n);
VRT = (5.00 / 1024.00) * VRT; // Conversion to voltage
Did you power the NTC+resistor with the 5V pin of the Nano ? If you accidentally used the 3.3V pin for that, then it will no longer be accurate. Do you use the GND wire to the NTC for something else ? If you, for example, have a single GND wire to a breadboard, and there is a OLED display and NTC on that breadboard, then it will be less accurate. You need a wire from the 5V pin and use that only to power the NTC+resistor, and you need a GND wire straight from the Arduino board to the NTC. Don't use that GND wire for something else.