According to the official website, the OP theme song of Sukitte Ii na yo.
was announced to be the late singer and song writer Okazaki Ritsuko
's song "Friendship", which was originally composed for Love Hina
. Okazaki passed away in 2004 at the age of 44 due to scirrhus stomach cancer.
Friendship by Okazaki Ritsuko
The first episode of Sukitte Ii na yo. will be aired on October 6th following a theatrical preview on September 29th.
Source: Sukitte Ii na yo. official website