Is battery enough to power the Atmega328, display, HX711
The output will be 5 volts is this enough?


SSD1306 minimum Vcc is 7 volts. So no, 5 V will not be enough. However if you have an SSD1306 module then it probably will be (check the documentation for your module to make sure).


  Is battery enough to power the Atmega328, display, HX711.


A single cell lithium-ion battery puts out between 3.0 and 4.2 volts depending on its state of charge. To get 5 volts from it you will have to use a voltage booster (AKA step-up DC/DC converter).