I really enjoyed DOl-OS even if I'm not big on puzzles in general.
It can make you frustrated at first if you go for Theophile's account, instead of the guest one, but a quick hitting of the 'H' key will solve it quickly (keep your curiosity in check though if you don't want to be spoiled/ruin the fun of solving puzzles).
Checking Theophile's entries was fun but short, concise and to the point. Which is not surprising considering their choice of profession. I like how you managed to convey that.
The writing style feels cold, calculated, almost robotic but with touch of human feelings, fitting perfectly with the theme and GUI animations. No noticable grammar errors. The puzzles themselves are thoughtfully integrated into the game, providing a seamless and logical progression.
One thing to be appreciated are the minigames serving as a nice break until you figure the next step out and as a time-waster too. Ah. And another thing too. If you pay close attention to the lower bar at the bottom of the screen, you'll notice that the time shown is the same as the one your phone or computer screen. It was a nice little detail which does add to the overall immersive amosphere.
Puzzle lovers are going to have a field trip with this one. The thrill of mystery and the inappropriate desire of going through someone's diary contents finding secrets are going to accompany them through their playthrough, alongside frustration and restarting a dozen times before getting things right.
Um... What I mean after the essay is: Well done! You managed to make a game that appeals even to those who avoid the puzzle genre like the plague.