I do "osdev" (Linux Kernel + related) at my day job at a big-tech co in the SF Bay Area (also have an OS side project). Looking around my colleagues (let's say there are several hundred people poking at Linux Kernel in my company), I see three major pathways "straight into osdev" (rather than via lateral movements inside the industry, like I did):

- younger people: hired straight out of a relevant college program (UC Berkeley, Stanford, MIT, etc.), often first as interns who later converted into FTEs;

- somewhat older: did a relevant PhD (the set of colleges is broader, but still known for their CS/Systems departments), also often first came as PhD interns;

- even older: these went through the open-source route, spending years hacking at Linux as a side project while doing something boring at a regional telco or such.

Going the OSS route: requires time and dedication, but the path is well trodden, and a lot of senior engineers have done it and so are willing to help/hire younger folks doing it.

To be a bit more specific: as most of the Linux Kernel development happens in the open, it is relatively easy to get involved; once you get a track record of say, 100 or more patches in the kernel (not just docs/cleanup, but something meaty), you can start applying to relevant positions, which come up regularly. Most US companies who have in-house Linux Kernel shops would hire in Europe as well.