Statement (A): Atomic weight of oxygen is 16 a.m.u.

Statement (B): One gram atom of oxygen is equal to 16g

Statement (C): 16 g of oxygen atoms is equal to $6.023\times {10}^{23}$  number of oxygen atoms.

Statement (A):

"Atomic weight of oxygen is 16 a.m.u."

This is correct. The atomic weight (or atomic mass) of oxygen is indeed 16 a.m.u. (atomic mass units).

Statement (B):

"One gram atom of oxygen is equal to 16g"

This is also correct. A "gram atom" (or mole) of any element corresponds to its atomic weight in grams. Since the atomic weight of oxygen is 16 a.m.u., one gram atom of oxygen equals 16 grams.

Statement (C):

16 g of oxygen atoms is equal to 6.023 × 10²³ number of oxygen atoms.

This is correct as well. According to Avogadro's law, one mole of any substance contains 6.023 × 10²³ entities (atoms, molecules, etc.). Since 16 g of oxygen corresponds to one mole of oxygen atoms (because its atomic weight is 16 g/mol), it contains 6.023 × 10²³ oxygen atoms.

Final Answer

All the above statements are correct.