Unlike the original story of the light novels, Oreimo Portable expands on the story, with complete story routes for Kyousuke and each of the main female cast written by the original creator of the light novels, Tsukasa Fushimi.
This game also has a Sequel.
Like many visual novels, Oreimo Portable presents minimal gameplay, with only "play" and "fast forward" controls usually present throughout the game itself. However, decision points appear on key parts of the story, requiring the player to choose through multiple choices to advance on the story. The choices the player makes will contribute to which story route he/she will advance to.
Aside from the visual novel layout of the game, Oreimo Portable also includes other elements which players will encounter in the middle of the game.
Two-shot Events are considered as mini-games in the middle of conversations with other characters (usually those who will end up with the player). This event is usually denoted by the words "Two shot start" in the middle of the screen. The player is tasked to decide on his/her reaction to the incoming pieces of dialogue the other character will say to the player: either "to reply" or "stay quiet".
When the player chooses correctly, the other character's opinion of the player will improve and it will contribute to the advancement of the desired story route later on. Otherwise, the player will receive an "emergency evasion" option; if the player fails to use this last resort, the story will finish at a bad end.
The game also has the O.R.E. System. Considered as an alternate to the traditional "multiple choice" gameplay in visual novels, this system allows the player to acquire special items called "Records". These Records can be used to manipulate the plot in a different way decision points advance the plot within the game.
The plot in Oreimo Portable acts as a spin-off to the light novels, which explores on the relationship between between Kousaka siblings Kirino and Kyousuke. The story begins the year after Kirino's infamous "life consultation" with her older brother, who accidentally discovers her darkest secret: that she is an avid fan and collector of anime and eroge involving older brother-little sister dynamics.
After Kirino returns from her trip to the United States, she and Kyousuke continue on the usual antics, with their friends Saori Bajeena and Kuroneko tagging along. At this time, Kuroneko is starting to become close to Kyousuke during Kirino's absence, and their relationship is used by Kuroneko to tease Kirino, who is her rival to Kyousuke, and — surprisingly — her best friend.
The player assumes the role of Kyousuke Kousaka, whose life as a mediocre person is changed by his 'life consultation" session with the little sister whom he has learned to ignore. The story later develops into Kyousuke's experiences with the different girls who have interacted with him, including Kirino, her best friend Ayase Aragaki and her classmate Kanako Kurusu.