Carbocations are formed in two major alkene addition reactions. In an HX addition reaction, the pi bond of an alkene acts as a nucleophile and bonds with the proton of an HX molecule, where the X is a halogen atom. This forms a carbocation intermediate, and the X then bonds to the positive carbon that is available, as in the following two-step reaction.
Similarly, in an  addition reaction, the pi bond of an alkene acts as a nucleophile and bonds with the proton of an  molecule. This forms a carbocation intermediate (and an  atom); the oxygen atom of  then bonds with the positive carbon of the intermediate. The oxygen finally deprotonates to form a final alcohol product, as follows.