Olyver poses as Loras Tyrell's squire during a sparring match. He makes flirtatious advances toward Loras and the two later have sex. Olyver discovers that Loras is due to be married, and relays this information to Littlefinger shortly thereafter. Olyver implies that noblemen in Loras' position make up the bulk of his clientele.
Olyver has been left in charge of Littlefinger's brothels, presumably replacing Ros. When Oberyn Martell and Ellaria Sand arrive to King's Landing, they attend the brothel, with Olyver showing three prostitutes to the couple. After Ellaria chooses one and dismisses the other two, Oberyn turns his attentions to Olyver, who protests that he's not on offer before giving in and admitting he's "wildly expensive". Oberyn asks if Olyver has ever been with a prince before and orders him to remove his clothes, grabbing him by the crotch. However, they are interrupted when Oberyn overhears a pair of Lannister men singing "The Rains of Castamere".
In the books, Loras Tyrell is left deeply mourning the loss of his great love Renly Baratheon, and famously swears that he will never love again, saying "When the sun has set, no candle can replace it." Several critics including Westeros.org were upset that Loras has sex with Olyver in "Kissed by Fire", because it seemed rather sudden, as if Renly's death did not deeply impact him. Writer Bryan Cogman, however, subsequently did an interview with Westeros.org, and specifically addressed this concern: changes in adaptation meant that Loras had to have sex with Olyver, to let it slip that he would marry Sansa, as part of the plot mechanics of the Lannister/Tyrell rivalry. However, Cogman explained, his earlier drafts of the episode actually included longer scenes which made it explicitly clear that Loras is not finished mourning Renly. Rather, Loras was very depressed and lonely, and in effect "drowning his sorrows" by engaging in casual sex in the hope that it would make himself feel better, but it ultimately made Loras feel more emotionally distraught and reminded of how much he misses Renly. Cogman lamented that "as we get closer to production a lot of trims have to be made and this scene was one where I had to do that," and he acknowledged that cutting the scene gave the unintended impression that Loras was moving past Renly, when he actually isn't.
In the books
In the A Song of Ice and Fire novels, there is no prostitute named Olyver. The character in the series assumes the role given another agent of Littlefinger, who obtains the information not from Loras Tyrell but from Sansa Stark herself.