To use the OLED is quite simple. Add this in the header of your Arduino script:
#include <U8x8lib.h>
// the OLED used
U8X8_SSD1306_128X64_NONAME_SW_I2C u8x8(/* clock=*/ 15, /* data=*/ 4, /* reset=*/ 16);
Add this in your setup()
u8x8.begin();
u8x8.setFont(u8x8_font_chroma48medium8_r);
Then to display data on the OLED, use something like this:
u8x8.setCursor(0, 7);
u8x8.printf("RSSI %d SNR %.1d", LMIC.rssi, LMIC.snr);