It is a common misconception that consuming dietary fat automatically leads to weight gain. The low-fat trends that emerged in the United States in the 1980s and 1990s were intended to promote heart health and reduce the risk of chronic diseases such as obesity, diabetes, and heart disease. However, it is clear that low-fat diets have failed. The rate of obesity has more than doubled since we started eating low-fat, indicating a significant public health problem.
Current research indicates that the type of fat matters more than the quantity. Not all fats are created equal when it comes to weight gain -- some fats, such as those found in olive oil, are healthy and may help control weight. A review of 12 randomized controlled trials published in the Journal of the American College of Nutrition in 2017 found that consumption of olive oil was associated with significant reductions in body weight, body mass index, and waist circumference compared to other types of dietary fat.
Clinical Studies on Olive Oil and Health
Important clinical studies have been conducted on humans to determine the impact of olive oil on health.
The PREDIMED study investigated the effects of the Mediterranean diet on more than 7,000 participants in Spain who were randomly assigned to one of three groups: a Mediterranean diet supplemented with extra-virgin olive oil, a Mediterranean diet supplemented with nuts, or a control group that followed a low-fat diet. Those who consumed a Mediterranean diet with olive oil had the most weight loss and reduction in waist circumference. 
The SUN Project is a prospective recruited more than 22,000 Spanish university graduates and followed them over time, collecting detailed information on their dietary habits, lifestyle, and health outcomes. In this study, it was found that a higher baseline consumption of olive oil was associated with a lower likelihood of weight gain. 
A study published in the journal Nutrients in September 2018 investigated the effects of olive oil polyphenols on 900+ participants with metabolic syndrome. They found that the consumption of olive oil polyphenols significantly reduced waist circumference. 
In 2022, a study was published in the Journal of the American College of Cardiology on 32,000+ adults in the United States that found that participants with higher olive oil intake had lower BMI levels than those who rarely or never consumed olive oil.  Remarkably, this study also found that higher olive oil consumption was associated with a lower risk of total mortality from all causes. Specifically, the study showed that for every 10 g per day increase in olive oil consumption, there was a 7% lower risk of total mortality, a 13% lower risk of cardiovascular disease mortality, and a 4% lower risk of cancer mortality.
Harvard University Professor Frank Hu, Director of the Boston Nutrition Obesity Research Center, states that it is a myth that olive oil promotes weight gain. In fact, he said, "There is no evidence that increasing olive oil consumption is associated with weight gain. On the contrary, higher consumption of plant-based MUFA such as olive oil and nuts may prevent weight gain and help weight control."
You can watch Dr. Hu's presentation on Olive Oil, Body Weight, and Longevity below.
1. Olive Oil and Body Weight. Systematic Review and Meta-Analysis of Randomized Controlled Trials
2. Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: a prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial
3. Olive oil consumption and weight change: the SUN prospective cohort study
4. Changes in Types of Dietary Fats Influence Long-term Weight Change in US Women and Men
5. Consumption of Olive Oil and Risk of Total and Cause-Specific Mortality Among U.S. Adults
6. Webinar (Oldways): A Spoonful of Olive Oil Makes the Vegetables Go Down: The Health Beneﬁts of Cooking with EVOO