People tend to be very busy on Ōmisoka because they have much to do to prepare for the new year, and New Year's Day in particular. Many even do a thorough house cleaning, called ōsōji (大掃除). The exercise is much like the annual spring cleaning that people in most colder climates do and even involves changing the paper on shōji doors and setting tatami mats out to air in the sun. Similarly, on the final day of school before winter break, elementary school children do their own ōsōji to get their schools ready for the new year, and most businesses spend the year's final work day cleaning. The purpose of all this is to get ready to welcome in the new year with everything—including people's minds and bodies—in a fresh, clean state, making everything ready for the new beginning that New Year's Day is held to signify.

After cleaning, Japanese have the largest dinner of the year. Around 11:00 pm on Ōmisoka at home, people often gather for one last time in the old year to have a bowl of toshikoshi-soba (年越しそば) or toshikoshi-udon (年越しうどん) together—a tradition based on people's association of eating the long noodles with “crossing over from one year to the next,” which is the meaning of toshi-koshi. While the noodles are often eaten plain, or with chopped scallions, in some localities people top them with tempura. Traditionaly, families make Osechi (おせち) for new year day because cooking during the first 3 days of the new year is not a good thing for Kami of kitchens. But nowadays, most of families buy Osechi, cook ordinary dishes, or just don't know that custom.

At midnight, many visit a shrine or temple for Ninen-mairi(二年参り) . (See Hatsumōde) (初詣). Ninen(二年) means two years and mairi means pilgrimage.

Another regular feature of Ōmisoka starts at 7:30 pm when public broadcaster NHK airs Kōhaku Uta Gassen ("Red vs White singing contest"), one of the country's most-watched television programs. Popular singers (and singing groups) split into two teams, women in the red team and men in the white, which then alternate while competing for the audience's heart throughout the evening. At around 11:30 pm, the final singer (or group) sings, and the audience and a panel of judges are asked to cast their votes to decide which team sang better. The winning team gets a trophy and "the winners' flag." The program ends at about 11:45 pm. Programming then switches to coverage of midnight celebrations around the country. But that custom is beginning to disappear. Young people are becoming less interested in the Red vs White singing contest. Nowadays, huge Mixed martial arts events also take place and many people watch those, instead.

Throughout Japan, Shinto shrines prepare amazake to pass out to crowds that gather as midnight approaches. Most have a large cast bell (see bonshō for photos) that is struck once for each of the 108 earthly desires believed to cause human suffering. The bells' tolling straddles the midnight hour, and their deep, low tones reverberate for miles through the crisp night air as they ring out the old year and ring in the new.

Widely unknown even in Japan is the meaning of the word Ōmisoka. The kanji 晦 written as ミ十 (mi-so) means 30, mi-so-ka (Kanji 晦日) refers to the 30th and last day of a month in the ancient lunar calendar. The prefix Ō (大) makes it a comparative, so it's the last of the last days of the month.