Why the fire is blue when there is enough oxygen?

There are many factors that determine the color of fire.

What is burning The light released by the fire is due to exogeneous chemical oxidation reactions, and therefore depends on the type of the material that is burning. Kitchen stoves use methane gas, which is rather different from the long organic chains in wood.

The temperature The temperature of fire depends on the type of material burning, the shape of the furnace/candle/etc., the convection conditions, availability of oxygen, etc.

See, e.g., here.

Very hot gas molecules glow blue. I guess you can do a simple dimensional analysis to understand why. Using Planck's relation for energy $$E=frac{h}{lambda}$$ where $lambda$ is the wavelength.

In the flame, you are seeing tiny particles that are hot, gaining energy from the reaction. The hotter they are, the bigger the energy is which means the smaller the wavelength is. Smaller wavelengths are related to the blue colour (big frequencies), while bigger ones concern red colour (smaller frequencies).

Yellow flames are due to glowing particles of soot that emit a black-body spectrum. These will only form when there is not enough oxygen in the gas mixture (incomplete combustion).

The blue emission comes from excited molecules that are produced in the chemical reactions.