Large numbers of oxygen isotopic curves of benthic foraminiferal tests from deep-sea sediment cores have been published. The curves are well-established reliable proxies for past climate and relative sea level fluctuations. In order to understand possible trends in the future climate, a precise identification of warmest events in the past interglacial records becomes a necessity. In this review, we have compared nine hitherto published high-resolution oxygen isotopic records of the last 420 thousand years in order to understand the intensity of the past warm events during interglacial periods. The rating of the intensity of the interglacial events as depicted by the oxygen isotopic variability is as follows; Marine Isotope Stage 5.5 > 9.3 > 11.3 > 1 > 7.5. This rating of interglacial warming is closely comparable with the standard oxygen isotope curve of deep-sea sediment cores and also to the hydrogen isotope curve of the EPICA Dome C ice core from the Antarctica. The remarkably high sea level during the warmest interval within MIS 5.5 reached about 7+ or -4 m above the sea level during MIS 1, and even possibly above the present-day sea level. The MIS 11.3 periods is distinctive as the longest warm period among the last five interglacial periods. This observation clearly suggests that detailed studies of MIS 5.5 and 11.3 are essential for the prediction of the future environment of the Earth under the global warming.