Is it possible to connect MAX30100 directly to ATmega 328p without Arduino? I could connect directly to the pins but I doubt the functionality as most of the projects used the sensor to connect directly to the Arduino board.
If cannot, is there any other suggestion of a sensor that can be used to measure heart rate and pulse oximetry?
Its possible, if you write most of your own code, say in assembler.
However, the forum would suspect that you really want to turn the ATmega328P into a bare bones Arduino and use the Arduino IDE and Arduino sensor libraries to program it.