Osamu Yamasaki, also known professionally as Yamazaki Osamu, is a Japanese animator, animation director, and producer born on February 19, 1962, in Kumamoto Prefecture. His entry into the animation industry began around 1980 when he moved to Tokyo, inspired by his senior from Kumamoto Prefectural Kumamoto Technical High School, Hiroshi Watanabe. After joining Studio Live, he studied under the influential director Toyoo Ashida, whom he regards as his teacher. Yamasaki further honed his skills at studios including Kaname Productions. In 1984, he co-founded the freelance studio Minamimachi Bugyosho with colleagues such as Masami Obari and Kenichi Onuki, which later incorporated with Yamasaki as its representative director.

Yamasaki made his directorial debut at the age of 24 with the original video animation series Yotoden, released by JVC in 1987. He served as the original planner and director for this period drama, which achieved significant popularity in OVA fan polls. As an original creator, he is credited for the OVA series Anime V Comic Rentaman. His other notable original works from this era include the OVAs Akai Hayate and Ankoku Shin Den Takegami, for which he provided the original stories and scripts. Beyond his own creations, Yamasaki has an extensive history of adapting existing manga and games. He directed the romantic comedy ItaKiss, based on the manga by Kaoru Tada, in 2008. He is best known for directing multiple television seasons and theatrical films of the Hakuoki series, an adaptation of the otome game by Idea Factory. His other major directing credits include Gallery Fake, the sci-fi epic Toward the Terra, and the fantasy series Spirit Chronicles.

Yamasaki is recognized for his versatility, but he is particularly noted for his skill in directing period dramas and stories appealing to a female audience. His work on Yotoden is credited with pioneering the historical anime genre for female fans, using beautiful visuals and handsome male characters. His direction is also known for a strong emphasis on layout and voice acting performance. Beyond his creative work, Yamasaki plays a significant role in the industry's labor movement. He is a former chairman of the Japan Animation Creators Association, an organization co-founded by his mentor Toyoo Ashida to improve working conditions and the status of animators. He has been a vocal figure regarding animator salaries and has worked to foster new talent as a lecturer at the AMUSEMENT MEDIA Academy, where he taught figures like screenwriter Mari Okada.