The points in support of oxygen 18 increasing is that vapor contains more 16 than 18. So typically, this doesn't affect the composition of the ocean because whatever is evaporated just comes back as precipitation and all is restored. However, in an ice age scenario, a larger amount of this precipitation is getting trapped as ice and not returning to the ocean.
This means that if 16 is leaving in greater proportion than 18, and it is not coming back as precipitation, the proportion of 18 eventually increases.