I would suggest trying 8.33 in Overwatch, this is matched to osu! using 75% monitor distance, which matches well with the play area of osu!
Set the Widowmaker (sniper) sensitivity to 45 to achieve the same match:
PS! I'm assuming you also play Overwatch with 1366x768 and with FOV 103, if not make the necessary adjustments based on these screenshots.