simple explanation how to save code that espruino run on start?

onInit will get called automatically, so no need to add E.on('init', function() { onInit(); });

Your main problem though is you're not initialising the oled display in onInit.

Try:

var g;

function onInit() {   
  I2C2.setup({scl:B10,sda:B3});
  g = require("SSD1306").connect(I2C2, function() {
   oled();
  });
}