This page refers to the adult, Osomatsu-San version of Osomatsu. Click here for the Osomatsu-Kun version, where the Matsuno sextuplets are 10-year-old fifth graders instead of NEETs.
Osomatsu Matsuno is the first and eldest son of the Matsuno Family.
Similar to Canon!Matsu, he's bullheaded and childish, yet he still has the nerve to present himself as the leader of the Matsuno sextuplets. He likes to think that he's the most responsible, much to his green brother's disagreement. From there, however, he diverts.
In The Place, Osomatsu is the most chilled-out brother. He's strong, and his strength is almost equal to a Chairman's... In general, he's quite the individual, so it's pretty hard to describe him...
"she's not wrong lol" ~O
...On your own Place-wiki? Really?
To learn more about the Osomatsu-San timeline, just watch the anime. There are around 25 canonical episodes, along with a few bonus ones, such as Episode 3.5, which you can find in the above link, along with Episode 26 (which has been rewritten under Osomatsu's request).