The couple made their red carpet debut at Vanity Fair's Oscars after-party Sunday night.
The Mad Men alum, 51, and Osceola made their red carpet debut as a couple while posing for pics at both the Vanity Fair party and the Mercedes-Benz viewing party Sunday night.
Hamm looked as dapper as ever in a striped, velvet tuxedo jacket and black pants, while the 33-year-old actress opted for something a bit bolder. Osceola rocked a multi-colored floral and geometric print long-sleeved dress, which featured a thigh-high slit, showing off her black, strappy sandals.
The couple is believed to have met in 2015 when Osceola appeared on the final episode of Mad Men, in which she played a secretary at the retreat Hamm's character, Don Draper, ends up at, but it's unclear when they started dating.
While Hamm and Osceola have kept their relationship pretty private, they have been spotted out and about over the last few years.
The pair was last seen enjoying some Super Bowl Sunday activities ahead of the big game last month and before that were spotted enjoying their time together while on an Italian vacation in September 2021.
Before coupling up with Osceola, Hamm was in a long-term relationship with Jennifer Westfeldt. The pair split in 2015 after 18 years together.
At the time, a source told ET that the breakup occurred a few months before the two announced the news, and that it was due in part to Hamm's "party-heavy lifestyle."
"Jon and Jennifer really fought hard to make their relationship work, Jennifer more so than Jon," the source claimed. "They were together for many years, but in the end, it was too much work. It's no secret that Jon likes to go out and leads a very party-heavy lifestyle. And Jennifer was just tired of that."
"She wanted him to grow up … and he never wanted to," the source added. "She was tired of being his mother."