It seems the temperature has been slightly hotter about 6500 years ago for a period of around 2 centuries with temperature estimated between +0.8 and +1.8 °C compared to 19th century, but this is subject to debate, (see for example https://www.nature.com/articles/s41597-020-0530-7).

Before that, we have to go back to a period where most Homo Sapiens were living in Africa about 125,000 years ago, where warming was likely +0.5 to +1.5°C compared to the same 19th century baseline.

Regardless if there was periods much hotter in the long past, the big difference with today's situation is the rate at which this warming is taking place. For example, for the "6500 years ago" period, it took about 3000 years of warming to go from +0 to it's maximum (which is between +0.8 and +1.8 °C). Today we are at about +1.1°C and it took us only 100 years, through fossil fuels burning and farming to reach that and most of which happened in the past 50 years.

Sources:

• IPCC WG1, figure2_1
• https://www.nature.com/articles/s41597-020-0530-7

Also, about oxygen 16 and oxygen 18:

The water remaining in the ocean develops increasingly higher concentration of heavy oxygen compared to the universal standard, and the ice develops a higher concentration of light oxygen. Thus, high concentrations of heavy oxygen in the ocean tell scientists that light oxygen was trapped in the ice sheets. The exact oxygen ratios can show how much ice covered the Earth. Sources:

• https://earthobservatory.nasa.gov/features/Paleoclimatology_OxygenBalance which is based on https://hal.science/hal-03334828/file/jgrD1994Jouzel25791.pdf
• You may also find this wikipedia article useful https://en.wikipedia.org/wiki/%CE%9418O#Extrapolation_of_temperature