Tea can be made from oregano, but it's not a pleasant drink. It's a wonderful antiviral/antibacterial though, if you can stomach it. I can't, so I use oregano essential oil in a capsule, so I don't have to taste it, that stuff BURNS going down!
Lemongrass is good, I'd also add horehound as a good cough remedy, lavender is nice for a relaxing bedtime tea, catnip is a mild lemony flavor that is also supposed to be relaxing (think Sleepytime tea), various mints, stevia makes a pretty good addition for a sweeter tea, you can use alpine strawberry (leaves and fruit) in teas, and marsh-mallow (not the white sugary kind, the flower) is nice for when you have colds/coughs. Rose petals and rosehips are nice if you like floral flavors, the hips are a little bit fruity tasting and also high in vitamin C.
Tea plants for growing white/green/black tea (they all come from one plant) are out there, but there's not many suppliers and they're rather pricey. I'd love to get some though. The problem is, you have to keep your hands off for a couple years at least so they can grow big enough.