char777

Regional crew who are located in bases with service for two or more of the majors can and do fly for all brands. In other words, if you're a SkyWest FA based in DEN, you might fly for UAX on DEN-MCI-DEN one day, then the next day for DCI on DEN-SLC-DEN. Sometimes the trips can be split up so that crew may work for two brands on the same trip (e.g. DEN-TUS on UAX, overnight, then TUS-SLC-DEN on DCI), but it's not common. Cabin crew have the same uniform but carry accent pieces (ties, scarves, etc.) for each airline, so an FA in these examples will have the same navy-black-ish jacket and pants but a red scarf for DL and that weird jumbled letter one for UA. I've been told that they also receive brand specific training for each of the majors for which they operate, though YMMV.

Planes are a little different story. Many of the regional carriers have aircraft purchased by the majors, but leased to, and operated by the regionals. I'm pretty sure SkyWest is one of the operators that owns most/all of its planes, including even the ones flown for AS. The dumpy 50-seaters change operators from time to time, but the nice new 76-seaters with the custom cabins seem to stay under the same brand for longer.

PV_Premier

there used to be a few CRJ 200s and maybe 700s around painted in OO livery. They would turn up primarily at ORD, DEN and SLC and I think they got used by both UA and DL. They're mostly if not all gone now.