Discoloration  

I can't seem to add a photo. So I charges my profile picture, where the discoloration can be seen. This is after sanding and before oil.

Photos need to be under 2MB I believe. It is hard to see from the small photo. If it is veneer and is stained all the way through the only way I know to make it less noticeable to bleach the whole thing and then try to restore the color once everything is more consistent and even then it may show through. Darker stains obviously work better to hide or blend things. If the veneer is nice and thick or if it is solid you can try using a wire brush to dig out some of the staining so the grain shows through better.

I will try again. It is veneer.

@hannekes What do you mean by discoloration? The darker spots on the left side or the color difference between the left and the right side?

Thanks for your reply. I mean the difference between the left and the right side.

Yeah, I'm not sure I would call that a discoloration, those are 2 different pieces of wood, they may have been close in color years ago but they have aged differently and what you are seeing it just a natural difference in the aging of the wood and stain. Tape it and apply a color to the lighter side to get it as close to the right as possible. Then remove tape and apply more color to both sides to blend. It doesn't need to match exactly as you can clearly see the grain is different where the 2 pieces of veneer meet.

Thanks for your reply. With what would you color it?

@HannekeS Dyestain in a teak color. (Is it teak? Or walnut? Hard to tell from the picture) If that turns out not dark enough, you can maybe mix it with mahagony. But you should start with a watered down solution.  It's easier to go darker than to lighten it up again. The problem with staining the whole top could be that it doesnt match the rest of the piece in the end.  Maybe @tktoo2 has something to say about it? He is more of a woodwoker than I am ...

You can use a ready made stain, there are plenty to choose from. I typically use a color base and dilute it and make my own when I want to blend color. You want to seal the wood first with something like spray lacquer and then apply the color on top. Oil based stains will take a while to dry on wood that is sealed but will still work. it may take several coats with an off the shelf stain to get the color you want. Some people like gel stains as they tend to be more opaque. You can experiment and simply remove it if it doesn't come out right (the benefit of sealing the wood first).

Hi, Either polish or paint with a primer coat will work. After the final coat of stain has thoroughly cured (typically overnight or 24 hours), apply polyurethane, or Tung oil. Apply at least two coats, since dining room tables get a lot of use, allowing each coat to fully cure before applying the next one. It is a small task and I hope you will be able to do it. Good Luck!

Looks to me as though the sideboard was originally a cabinet with a bookshelf on it. That would explain the alternative veneer to me, the left side definitely doesn't appear to be teak, so as all have suggested here you can try to colour match with dye stain but it'll be a job getting it just right. And even the the refractive properties of the wood may differ, so the colour may not match in all lights