can you gain weight using olive oil?

Of course you can, but it does not have to be.  Olive oil, like any other fat, has 9 calories per gram.  If you're substituting the fat you currently use with olive oil, you're not adding any calorie and your weight will remain stable.  If your ADDING olive oil on top of what you're already eating, or if your using MORE olive oil than before, you risk gaining weight.  How much weight? depends on how much olive oil.

Balance.... calories in and calories out.  Olive oil is a good thing, in moderation.