In Romeo's line, "O, I am fortune's fool!," the term fortune can refer to fate as well as to general prosperity (III.i.138). The word fool can refer to a very stupid person or to a person who is being kept for entertainment purposes, like a court jester. Hence, in this line, Romeo is either most likely saying that he is "fate's court jester," or "prosperity's court jester." The events leading up to his realization that he is being used as a court jester are that he finally finds true love, marries Juliet, and then is faced with a battle with her cousin, leading to Mercutio's as well as Tybalt's deaths.
When we first meet Romeo, we see that he feels very down on his luck concerning love. In fact, he is heartbroken to the point of distraction due to Rosaline's rejection of his love. The moment he meets Juliet is the moment he starts feeling prosperous in love because not only does he now love Juliet, she actually returns his love, unlike Rosaline. Fate or prosperity further enriches Romeo by providing him with the opportunity to marry Juliet in secret with Friar Laurence. However, the moment after the wedding, Tybalt, having felt insulted by Romeo's presence at the ball, comes in search of Romeo to challenge him to a duel as compensation for the insult. Romeo, on the other hand, tries to pacify Tybalt, knowing that Tybalt is now his own cousin through marriage. However, Mercutio becomes inflamed by what he perceives to be Romeo's "calm, dishnourable, vile submission!" and answers Tybalt's challenge himself, resulting in his own death (III.i.72). Suddenly, Romeo is now faced with the need to avenge his friend's death on his new cousin Tybalt, resulting in Tybalt's death and Romeo's separation from Juliet through banishment. Since fate has just taken Romeo from an extreme emotional low to an extreme emotional high and then back again to an even worse emotional low, we can understand why Romeo feels that either fate or prosperity is using him for its own personal joke, making him "fortune's fool."