How do oxygen isotopes help scientists extract temp info from ice cores?

Oxygen isotopes solely from ice core samples would tell you that +/- in temperatures and ice volume occurred individually or together. So they are used as a comparison against other oxygen isotopes found in carbonates, and the volume of ice compared to sea levels at different historical periods. During periods of low sea level such as the last ice age, the lighter isotope is stored in the ice making the oceans isotopically heavier.