OSTEP Textbook is *phenomenal*. I used it to study for job interviews way after the course was over (and did get an offer from one of those interviews lol).
Reiher is a great lecturer, but even more important, his slides are great for studying and referencing during lecture. Projects are pretty time-consuming, comparable to most other project-based CS classes, but TAs are incredibly helpful during discussions, showing us how to set up the skeleton, what proper functioning looks like, etc.
Would take again with Reiher 10/10

Reiher is a no-nonsense professor. He gives his lectures in a very straightforward and matter-of-fact style, no jokes or humor or anything unrelated to class content. So I wouldn't exactly call him an engaging lecturer, but he is very articulate and clear, while also going through material relatively fast. His lecture slides are detailed and contain most of the information you need.

The workload in this class is tough, I think it deserves its reputation for the most part. Every week there is a project, most of them take 10-20 hours. In addition, expect to sink another 5-10 hours per week into doing readings and taking notes on them. The key to success in this class is taking meticulous notes on everything.

The difficulty of this class stems from the workload and breadth of material, not the conceptual aspects. There is comparatively little critical thinking when it comes to the tests. Doing well on those mostly comes down to how well you retain the readings or lectures (or how good the notes you took are). Overall, probably the best you can expect for 111, as other reviews have said.

Excellent professor. Best you can ask for in CS 111. Speaks clearly during lectures, covers topics in a timely manner, and allows students to ask questions whenever they desire. Readings are thiqq but they're the same with every professor. Same deal for the projects. Online tests were tricky because they were multiple choice with "select all that apply" types of questions, but the topics were always covered in lecture/readings. Take Reiher if you can.

I took this class online during spring quarter, and boy oh boy were the all-new CCLE timed multiple choice style tests a doozy in comparison with the standard all-free-response style midterm and final that is usually administered. Study hard and study well, because you'll be faced with mostly "Select all of the following that apply" type of questions with options a-f, and every little detail counts. The nice thing is that Professor Reiher gives out grades based on natural breaks (like how Corbin grades his physics classes), so many people who got low scores ended up with good grades in the class.

In terms of the projects, the TAs for the class graded much more leniently than the TAs for 35L, and you get to test your code with a "sanity script" that supposedly only checks for barebones functionality (although for certain projects, I'm pretty sure the TAs just use the sanity script to grade). I personally thought the projects were easier than 35L (especially compared to Aaron's 9th week project if you know what I mean), but don't be fooled into thinking you can breeze through them.

Also remember -- don't fall behind on the readings.

This class should be two classes. CS 111 had the most readings of any course I've taken at UCLA, and the projects were very unforgiving. The only saving grace for this course was that the midterm and final were fairly manageable so long as you did the readings, but there's so much to cover that you'll likely lose points here-and-there depending on the strictness of the graders. Reiher is also a good professor so I recommend taking CS 111 with him if you can. Also, don't trick yourself into thinking that taking it over the summer can make it easier. This may be the case with other courses but for us we had project due dates packed so closely together that the 2nd to last project was due the day of the final exam and the last project was due less than a week later with a hard deadline.

tl;dr CS Major Beware, You're In For A Scare

Despite hearing he is easier than other professors teaching this course, I still found the workload to be a ridiculous amount. 100+ pages of dense OS reading every week, alongside large projects that have nothing to do with the lectures. It's like they are two different classes. Professor Reiher tried his best to squeeze a course designed for a semester system into our very own rubbish quarter system, and the students pay the price. It's like CS kids aren't allowed to do anything besides study lol

Reiher is definitely a lot easier than the other professors. You don't have to attend lectures as he reads straight off the powerpoint. There is a ridiculous amount of work though, 100 pages reading a week at least and a 10-20 hr project. Good luck.

You get to learn a lot of cool stuff about how a computer really works on a lower level in this class, and how the abstractions we use on a daily basis are constructed - which is what made this class really fun for me. With Reiher, you HAVE to know your content in and out because the tests really check if you understand each and every concept and you can't get away with just kinda knowing it. That said, Reiher is a great professor: he may not be the most engaging, but if you manage that - just keeping up with lectures and readings should be good for this class. Also, projects are a pain - get started on them as early as possible (they do get easier towards the end of the quarter though).

Reiher is one of UCLA's top notch professors. Always open to answering students' questions and gave engaging lectures with some humor thrown in here and there. Even though most of us won't go into OS development (or debugging for that matter), this class teaches you how computers work behind the scenes to make applications run smoothly together (e.g. resource sharing), how the most vital piece of systems software interacts with the hardware to create the abstractions us programmers often taken for granted, and most importantly develop a systems mindset for thinking about software. Projects were time consuming so please start as early as possible; tests are quite unpredictable tbh could be very detail oriented or require tying together general concepts. The only real way to prepare for his tests is to do the readings AND take notes of the key ideas. The difficulty of this class is over-hyped it is definitely manageable if you put in enough effort and time. From what I heard, Kampe is a bad lecturer and Eggert is Eggert so... better to take it with Reiher. If you're a HSSEAS student and plan on tech breadthing in CS, definitely take 111 as this is one of the core classes that distinguishes between a programmer and a software engineer.

Definitely take this class with Reiher. It's time consuming and the projects are hard, but that will be the case with any professor you have for this class.
There's a lot of reading so definitely do it each week as he assigns it. When the midterm and final comes along it's just way too much material to cram for.
Lectures are entirely slide based, so feel free to skip them and review it on your own. Lecture material closely parallels the assigned reading that week, but there are some topics in the lectures but not in the readings (and vice versa) that comes up on the tests so you absolutely need to go over both.
Projects are really difficult, the biggest help was my TA (Jonathan Lin, highly recommend) during discussion section and office hours. I would go around and check out each TA, some explain things better but others have sample code which is super helpful.
Reihers late policy is similar to Eggerts but a little harsher (something like 2^(n+2) iirc) so there's some wiggle room, but keep in mind you have a project due every week so it's best not to fall too far behind.
The tests are the biggest reason to take this class with Reiher. The midterm and final both consist of writing answers of around 5 sentences for questions like "What is password salting" or "What scheduling algorithm best prioritizes _?" Some might be tougher but in any case, reviewing reading and lecture notes can definitely get you a solid grade on the exam. It's much easier and helps you learn the material more.