I always had Sinus issues as far as 12 years now. Allergies manifested as really quickly. Oil pulling is a god send for people with any of these issues. It became a routine for me before brushing and makes my day a whole lot better. It loosens up the phlegm in the sinuses that accumulates overnight and make it easy to sneeze it out. It seems to also take care of the stubborn plaque and teeth cleaning. Sometimes, if the brushing in the evening does not make me feel clean on teeth, i get to another round of oil pulling to clean up.
However, do not oil pull without at least 3-4 hours after eating and 30mins after drinking anything. This is also mentioned elsewhere. Please follow the instructions meticulously. One thing i noticed is that the amount of time you swish it in mouth depends on how stuffed you are. If you are good, oil takes a while to liquify and if you are stuffed, it liquifies quickly and you can feel it thinning.
This also helps me manage my allergies and I stopped using the sprays and now i am on accupucture treatment to completely get rid of these problems.
If you have that feeling of phlegm stuck in your lower sinuses even with oil pulling, you may want to use netipot. Use kosher salt and warm water. That should give you relief for atleast 24 hours, free of any stuffing what so ever. There are advanced techniques with neti pot (drug stores call it sinus rinse) which can effectively clean every part of sinus over time. Good luck!
EC: More about Oil Pulling here: https://www.earthclinic.com/remedies/oil_pulling.html