What type of oxides are formed:
(a) when metals combine with oxygen?
(b) when non-metals combine with oxygen?

(a)  Metals react with oxygen to form basic oxides.

Example: Sodium reacts with oxygen to form a basic oxide called sodium oxide.

(b) Non-metals react with oxygen to form acidic and neutral oxides.

Example: When carbon reacts with oxygen it forms carbon dioxide, which is an acidic oxide, and water is formed when hydrogen reacts with oxygen, which is neutral.