Can dark mode saves battery

on an OLED display a bit

Hello,

Its possible that it could but I am unsure of the main color of your screen actually affects battery usage or if the backlight (brightness) is the only display setting that affects it.

Batteries - Maximizing Performance - Apple

"Optimize your settings.

There are two simple ways you can preserve battery life — no matter how you use your device: adjust your screen brightness and use Wi‑Fi.

Dim the screen or turn on Auto-Brightness to extend battery life.

• To dim, open Control Center and drag the Brightness slider to the bottom.
• Auto-Brightness adjusts your screen to lighting conditions automatically. To activate it, go to Settings > Accessibility. Tap Display & Text Size, then turn on Auto-Brightness.

When you use your device to access data, a Wi‑Fi connection uses less power than a cellular network — so keep Wi‑Fi on at all times. To turn on Wi‑Fi, go to Settings > Wi‑Fi to access a Wi‑Fi network."

I hope this helps.