This is unlikely. You need to count your calories and stay under your daily calorie needs in order to lose weight, consume the number of calories equal to your daily calorie needs to maintain your weight and consume more than your daily calorie needs to gain weight. It is very unlikely that eating olives would cause you to gain weight unless you ate a bunch. http://www.bmi-calculator.net/bmr-calculator/.
Not typically. To gain weight from fat in olives one has to consume significant olives.
If you eat a lot. The fat in olives has the same number of calories per gram as the fat from eggs or meat. You have to look at your total daily calorie intake.