3.1-kun (Japanese: 3.1くん, 3.1-kun), also known as Oojii (Japanese: 大爺様), is an OS-kun that depicts Windows 3.0 to 3.11. He is one of the lesser known OS-kuns.
3.1-kun is a man who dresses in green Meiji-era Military attire with a "3.1" medal on the right side. He is depicted as the younger brother of 1.0-tan and 3.1-tan, the older brother of NT-Mama, and the uncle of 95-tan. He speaks in the tone of an old man. Unlike 3.1-tan who has DOS kitty, 3.1-kun has a DOS cat girl.
After the OS wars, he retired with KanjiTalk 7.5-tan. Occasionally, he would test his skills with 2K-kun and XP Pro-kun, but he rarely appears.
- Windows 3.x, page about the OS itself and its family, that also lists alternate personifications.