Put simply, Altitude. At 8000m the human body really doesn’t get enough oxygen to survive. So any time you climb a mountain at this height you’re really doing so on borrowed time. Hypothetically, if you were to be picked up and dropped at the top of everest with no acclimatisation, you’d survive for a few minutes before passing out and then dying.

Anything about 8000m is considered the Death Zone. Here the air density is so much lower than on the ground, that your body gets just 34% of the amount of oxygen it normally does.

Kilimanjaro is considered harder than Everest base camp. There are three observable differences that make it so:
Summit day on Kilimanjaro is harder than anything on the Everest Base Camp trail
You go to altitude much quicker on Kilimanjaro. Everest base camp is a gradual 12-13 days trekking, but even though the days are longer, there is more up and down than on Kilimanjaro The success rate for Kilimanjaro is about 65%, which Everest Base Camp is up to 90%

n short, no. Most trekking companies will bring supplemental oxygen as a precaution, however using it can actually be more dangerous than not! Exposure to altitude can bring about altitude sickness (Pulmonary or Cerebral Edema- water in the brain and lungs respectively), and the side effects are headaches, nausea, shortness of breath. If you use oxygen after you’ve already developed the condition, and continue climbing, the illness may get worse but without you noticing. This could be quite dangerous, and descending the mountain should happen immediately, rather than masking the symptoms with supplemental oxygen, as oxygen alone cannot cure the illness.
This being said, the fatality rate on Kilimanjaro is very low, so oxygen is not something to be overly concerned with, trust your guides. On other mountains, oxygen is standard

There are so many easy mountains of equal difficulty that this is hard to answer. Mount Kosciusko is the lowest and by far the easiest of the seven peaks, for example, but as it’s located in Australia it might be better to attempt a similar or lower height mountain closer to your home. Ben Nevis is England’s highest peak and is only 1345m, and can be summited in 3-5 hours comfortably, with no technical ability required at all.

The 7 peaks challenge is the attempt to summit all of the highest peaks on each continent, 7 mountains total. As this includes Mount Everest, there are only approximately 416 people who have completed it. There are 2 lists of highest continents, the Bass list and the Messner list, the latter includes Mont Blanc, making 8 mountains of which only 30% of these people have completed it, approximately. The highest peaks on each continent are:

The 14 peaks are the only 14 mountains with >8000m summits on Earth. There is also the 14 peaks challenge, which consists of 14 900m peaks in Wales, not to be confused!

Cho Oyo is regarded as the easiest 8000m peak. This is because there are fewer random risks to deal with, such as a lower avalanche and ice fall risks and lower technical difficulty. However this shouldn’t be confused with an ‘easy mountain’, any 8000m peak is by no means easy, and Cho Oyo is the 6th highest mountain in the world.
What is the hardest mountain to climb in the world? The hardest mountain to climb in the world is Mount Annapurna, followed closely by K2. Annapurna has a whopping c.40% fatality rate and takes 40-50 days to summit.