What would be the mass in amu of 150 atoms of carbon?

Do you mean the periodic table? The atomic mass of carbon is 12 and oxygen is 16.

If I understand you right though, what you’re looking for is what oxygen’s mass would be in a certain unit of measurement with which carbon weighs 150 units.

In regular atomic mass units carbon’s mass is 12 and oxygen’s is 16, so oxygen’s mass is 4/3 times carbon’s mass. In our new unit of measurement where carbon weights 150 units, oxygen would weigh 150 * 4/3 = 200 units.