Ojisan is Cheering for My Love (In My Brain)

Matsuri Miyashita (26) is an ordinary OL with a crush on the handsome prince of a partner company. Seiichiro Kiyokawa (52) is an experienced salaryman with 30 years in the sales department, a devoted family man. On this day, the lives of these two unrelated people will come together in an unfortunate way!

"Why is there a strange old man's voice inside my head?"
"Why do I look like a girl?"

A triangle love comedy between OL x salaryman x hunk!

source: https://www.mangaupdates.com/series/xiyo1ft/wake-atte-oji-san-ga-watashi-no-koi-o-ouen-shite-imasu-nounai