An oxygen atom in a molecule can carry a partial charge, but it is not typically fully charged as either positive or negative. The charge distribution within a molecule depends on the electronegativity of the atoms involved and the overall molecular structure.
Oxygen is more electronegative than many other elements, such as carbon and hydrogen, so it tends to attract electrons towards itself in a covalent bond. This leads to an uneven distribution of electron density, resulting in a partial negative charge on the oxygen atom and partial positive charges on the atoms it is bonded to.
For example, in a water molecule (H2O), the oxygen atom attracts electrons more strongly than the hydrogen atoms. As a result, the oxygen atom carries a partial negative charge (δ-) while the hydrogen atoms carry partial positive charges (δ+). This partial charge distribution contributes to the polarity of the molecule.
It's important to note that the overall charge of a molecule is typically neutral, meaning the sum of all the partial charges within the molecule adds up to zero. This occurs because the charges are distributed among the atoms in a way that balances out the overall charge of the molecule.