Question

what is meant by saying that the molecular mass of oxygen is 32'

Answer 1

The mass of the molecule oxygen gas O2 is 32u. Atomic mass of of oxygen O is 16u and since O2 is molecule therefore molecular mass of O2 is 16*2 = 16u.

Answer 2

oxygen is diatomic molecule O2 as each molecule consists of two atoms

for example: a molecule of O2 consists of two oxygen atoms bonded together (O-O)

atomic mass O = 16
molecular mass O2 = 16x2=32

Note: there are only 7 diatomic molecules:
N2
O2
H2
F2
CL2
Br2
I2