I prefer O'Reilly over other parts chains, perhaps my employment there gives me some bias. I've never heard of "one register reserved for checkout" - that is an interesting concept, but without enough staff even that would be a problem. I've been alone in the store for several hours at a time, which I can usually handle (it becomes an issue when the phone rings, or if people want help outside with wipers or battery testing or whatever,though). Even with two people, if one was looking up parts and one was just cashiering, when a single complicated parts lookup can take several minutes, I'm not sure how well it would work for other people that want parts - though it would be good for the people that just wanna buy car wash stuff.
As for staffing, y'all are right, parts store jobs aren't really a career, unless you're at least a store manager in the right store and the job is the right fit for you. Store manager at my location has been with the company for I think, 6 years, 9 years? I don't remember. Anyway, she likes her job, and it has done well for her - she just purchased a house with her husband (who is a mechanic at a local auto shop), and they have two nice cars. Assistant store manager or below, though, just a typical high-turnover low wage retail job. I'm not bashing O'Reilly - they treat their people well, the benefits are good for full-time employees compared to other retail jobs, but the pay is just too low to make a career of it.
I mean, I have a few coworkers that have been there for a long time, I think one of them even worked for Grand Auto (acquired by Kragen, which was then later acquired by O'Reilly)... and several others that are from the Kragen days. None of these people are particularly motivated or brilliant individuals. The Grand Auto guy hasn't turned a wrench in his life, "hates cars" and while he has one, he never drives it - he takes the bus, the taxi, or pays me to drive him home.
But we're always short on staff. Most people with interviews don't even show up or even call ahead to say nevermind I am not interested anymore or got a job somewhere else. How rude. And people are leaving all the time. Many part-time employees that have to fit their schedule around other jobs or school or family stuff.
All that said. If you have any interest in cars, get a part-time job at O'Reilly. At least around here literally every store needs employees, so just apply online, and if you pass the background check and drug test and you're not a totally bad person you'll probably be offered a position. The parts discount is very good and starts on your first day (though oil is still cheaper at Walmart) and if you do need to take your car to the shop you'll find a shop you deliver to that can give you a good deal.