Antonyms for the phrase "give the onceover" are challenging to find, as it is commonly used to describe a quick, cursory examination of something or someone. However, opposite phrases might include "ignore thoroughly" or "take no notice," which imply that no attention or focus is given to the subject at hand. Other antonyms might include "deeply analyze" or "thoroughly scrutinize," which suggest that a more in-depth examination or evaluation is necessary. Ultimately, the antonyms for "give the onceover" depend on the context in which it is used, as well as the degree of scrutiny or attention required.