A book with that exact title doesn't ring a bell for me as a major published work. If you're talking about the phrase "everything is OK" as a theme, there are a few possibilities. There's a memoir titled 'Everything Is OK' by Debbie Tung about anxiety and mental health, which is quite lovely. For that one, your best bet is likely through mainstream ebook retailers like Amazon Kindle, Apple Books, or Kobo. Libraries often have it via apps like Libby as well.
If you're thinking of something else, maybe a web serial or a self-published piece with a similar name, the search gets trickier. Sites like Wattpad or RoyalRoad are packed with original stories, and the title could be a chapter name or a tagline rather than the full book title. I'd suggest searching the phrase in quotes on those platforms. Without a definite author, it's a bit of a needle in a haystack situation, honestly. I once spent an afternoon looking for a vaguely remembered novel only to find it was a fanfiction with a different title on Archive of Our Own.