It has a charge pump to provide the panel voltage.
2 Charge Pump Regulator
The internal regulator circuit in SSD1306 accompanying only 2 external capacitors can generate a
7.5V voltage supply, VCC, from a low voltage supply input, VBAT. The VCC is the voltage supply to the
OLED driver block. This is a switching capacitor regulator circuit, designed for handheld applications.
This regulator can be turned on/off by software command setting.