Raspberry Pi OS, previously known as Raspbian, is the official operating system for Raspberry Pi devices. It’s based on Debian and is optimized to run on the Raspberry Pi’s hardware.
How to run Pi OS
Step 1 : Download Pi OS Image file from official website, https://www.raspberrypi.org/software/. And install pi OS in SD card as bootable.
Step 2 : Insert the SD Card into the Raspberry Pi’s microSD slot.
Step 3 : Connect LCD Display to HDMI port or GP pin.
Step 4 : Connect mouse and keyboard to USB port.
Step 5 : Connect the power supply to Raspberry Pi.