The atomic mass unit (amu), also known as the unified atomic mass unit (u), is a unit of mass commonly used to express atomic and molecular weights. The standard definition of the atomic mass unit is based on the carbon-12 isotope.
The current standard defines one atomic mass unit as 1/12th the mass of a carbon-12 atom. This definition was established by the International Union of Pure and Applied Chemistry (IUPAC) and is widely accepted in the scientific community.
If you define the atomic mass unit as 1/16th the mass of an oxygen atom instead of 1/12th the mass of a carbon-12 atom, it would deviate from the standard definition. While you can define your own unit of mass, it would not align with the commonly accepted scientific conventions and could lead to confusion and inconsistencies when communicating with others in the field of chemistry.
It's important to note that the choice of a standard unit like the atomic mass unit is based on practical considerations and conventions agreed upon by the scientific community to ensure consistency and comparability of measurements.