Well, it depends on the tree and how old it is.

live oak...not sure how old but really big, i would guess about 30 years. grass refuses to grow under it, but that is most trees.

I've not ever been successful getting anything but annual salvias, coleus, weeds and grass to grow well under established live oaks. After years of trying to have that pretty planting ring around our live oaks I finally gave up. Removed the ring and let the grass have its way. You might have better luck.

If the tree is older and established there isn't much loose soil to plant in. You do not want to pile up fresh soil to plant in. Doing that will kill the tree. If you want that pretty planting ring around live oaks, you have to start it when the tree is young and plant perennials that can establish themselves while the live oak is putting down its big roots and before it starts on those shallow roots. But eventually the live oak will send up its suckers and you'll be clipping them out through the other plantings.

If you insist on that ring around the tree, maybe plant some ground cover and place pots in it with your colorful plants and flowers in them.

A lot people don't like Ruellia because it can be invasive, but it grows well around the bottom of one of my Oak trees. If it comes up where I don't want it, I just pull it out or keep it mowed.
It's the tall purple one, but doesn't get over about 24" for me.
My yellow Columbine is under another Oak tree and does well.

I have columbine, Ajuga repens and salvia romeriana. They all bloom at the same time and it is a palette that was colored in by a child ...BRIGHT PURE CRAYON COLORS. No subtlety but it makes the child in me laugh. They handle the oak tre well. These are small live oaks. No leaves right now. Aloe and dykia grow well there also. I do have to protect them in the winter.

I had Chi Chi ruella under my oaks at my old house. They were invasive.I coupled them with Greek pattern plant, mexican turks cap, Tahitian bridal veil (another relentless spreader but it was pretty under the trees. I moved away from the invassive mix I created but both neighbors loved my choices and were not gardeners so they loved that my plants moved in and made their bare under tree dirt look good. LOL

Everyone is correct. It's the perfect spot for those pretty, but incredibly invasive plants, that tolerate shade. Under my Elm and Hackberry tree corner I have ruellia, iris (they really don't need a lot of dirt to set in but they may not flower), bouncing betty, obedient plant, and side oats gramma grass.

Thanks for the input. I do already have iris underneath and bonus, I just got them in the fall and would you believe they are blooming? I also had some varigated liriope that desperately needed thinning and put some of that underneath. Not terribly exciting, but I know will do well (after reading some of your post earlier). Put in some holly ferns, which I know is a gamble, but remembered they did well under our mature cypress tree at the last place.
Any idea if vina minor ( the one that trails) would do well there? I have a ton of it in a full sun spot that need to find a new home.
I love the idea of obedients and bouncing bet. I love both of those. I am bummed neither of them survived the move. They were never invasive for me, but I had heavy clay at my last place, so they never wandered much.
Oh and I have some lyre leaf sage starts that are doing well there too.
Spent all day recycling plants and adding a few purchases.
Added a rock border and raked up all of the old leaves to use them as mulch.
I will try and post a pic soon.
Still have some spots to fill for sure, but feel good about getting a head start on this space.
Thanks for all of the great insight and ideas!

The vinca minor will do well in the shade. The variegated variety is particularly pretty in the shade. I second the yellow columbine. If you find one in bloom this next month, ask for some seed. It produces a lot. I don't know how easy it germinates, but it likes dry shade. I would try salvia too. I think the red ones throw off seed pretty well.

You could look for woodland violets, they make good ground cover with nice looking plants most of the year and tiny bloom in early spring and the cost is the best, nothing but the transplanting.

I have pigeonberry under an oak tree. Never seen seeds for sale but they will reseed once you get some started. It's not a real bright plant, but it makes a nice groundcover.