Hello. I need help color matching polyurethane. In the photo, I am trying to finish the right side to match the deep, golden color of the left side.

The left side was finished with Parks Pro sanding sealer and poly. The right side currently only has Parks Pro sanding sealer applied. I don't know how the color on the left side was acheived. I've been trying to color match the poly. I've gotten closest by mixing the poly with burnt sienna oil paint, but that comes out too redish.

Does anybody know how to get this color?

depends on how long ago the left side was done..if its been few years and the finish ambered, you will just have t wait the curse of time..otherwise, you would need to sand and try a light stain like minwax golden pecan,or natural stain with a mix of gold pecan or golden oak..it takes experimenting..

if the floor was a recent finish, than just apply the poly and see if it deepens the sealer

Here's an updated photo of the first one I posted.

Left: Finished floor that I am trying to match.
Middle: Previously coated with Parks sanding sealer. Over the dried sanding sealer, I brushed on Cabot Colonial Maple oil stain and let it sit for 10 minutes. I then wiped it off.
Right: Parks sanding sealer only.

I'm getting a bit closer, but I'm still not there yet. The color seems right, but it's not dark enough.

Ideas?

Looks to me like ambered finish on the left. If it's natural it'll be very hard to match. If you do match it, the right side will likely be darker in a year's time after it starts to amber.
I'd use the same finish that's on the left. At first it'll be lighter, and you'll see the red in the red oak, but in time it will match . Best way to match is sand the whole room. I'm guessing with the time you've spent staining and finishing and re-doing you could have had the whole room ready for a coat by now.

Amber shellac. Try it on a sample first. Works like a charm!

But Parks Universal Sealer is dewaxed amber shellac, isn't it? Adding more coats of Universal Sealer doesn't make it darker. What am I missing here?

Just finish it the same way you did the other floor. If you get it to match now when it changes naturally it will be different forever. Following the same procedure is your best chance shot of sanding and finishing everything together.

You originally posted 3 weeks ago, could have been done and living on it by now.

the easiest way to achieve good result is re sand the whole thing and start all over again from primer to top coat...suggestion try bona kemi... either bona mega or bona traffic...