I have been downloading C-Ost in Youtube ever since kdramaost.com is not updating.
I use https://savemp3.net/3qivu/ - for downloading (Just copy the link of the song you want to download from Youtube then paste the link in savemp3).
Then I use https://tagmp3.net/ - for free online mp3 tag editor, just click choose files then search in the downloads folder your downloaded song from youtube). Example: Black Moonlight ost version of Bai Lu & Luo Yunxi - Till the end of the moon. Then fill up the following once you already choose an ost: Choose new album art / picture ( I just download photos here in MDL - this is for the cover photo you want to use in the Album), Just click browse. Then fill up the title: Black Moonlight, Artist: Bai Lu, Leo Luo, Album: 长月烬明 Till the End of the Moon.
As for the rest Track Number, Genre, Comments, Year ( I don't usually fill this up).
Then click Done! Generate New Files down below
Just A reminder: If the title from Youtube have a comma, after downloading the song kindly edit out the comma and change it from "and/ &". For example: 黑月光 (Black Moonlight) - 白鹿 (Bai Lu), 罗云熙 (Leo Luo). Usually https://tagmp3.net/ will show ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_DISPOSITION; - this means you have to remove the comma and you will be able to edit the song successfully in the tagmp3.