Unequal colour can be due to several things:
1. Unequal sanding
The smoother the surface, the less oil can penetrate in the wood and the less intense the colour will be. So maybe you haven't sanded the whole surface the same way or you didn't change the sanding paper regularly and because of that the paper was already worn by the end. This means you sanded the last parts much smoother than in the beginning, resulting in a different colour.
Tip: if you're working with an orbital hand sander, make sure you're not pushing too hard and you're not working on the highest setting.
How to fix it? Unfortunately the only option to fix this is by re-sanding the surface until the whole treatment has been removed and then to re-oil.
Tip: After sanding, use WoodPrep. This product will help to minimise sanding errors. The product avoids (or reduces) potential for visible sanding swirls, provides a more consistent and even colour penetration with an uneven finish sanding and a more consistent colour penetration on naturally blotchy wood species (e.g. maple).
2. The excess was not removed sufficiently
It's important that once the oil has made the molecular bonding, you remove all the excess. Otherwise this will result in a sticky non-equal coloured surface that is prone to scratches. Make sure you regularly change the white pad or the cotton cloth you're using to remove the excess.
How to fix it? Use a maroon Rubio Monocoat GS320 pad to remove the bit of excess there might be left on the surface. Go over the whole surface and equalise the colour. Don't forget to change pads regularly. Once you have obtained an equal colour, vacuum thoroughly and re-apply a bit of oil. The consumption will be a lot lower than the first time you treated the surface.
3. The oil wasn't mixed well enough or you used different containers for one job
After a while, pigments will sink to the bottom. Always stir the oil well before use and regularly during application.
How to fix it? Unfortunately the only option to fix this is by re-sanding the surface until the whole treatment has been removed and then to re-oil.
4. Different pathces in the wood
Differences in colour can also be due to the different patterns in the wood. Lighter and darker patches are perfectly normal, it's simply one of the natural characteristics of wood. Oil is a always a transparent finish so it won't hide these differences.
How to fix it? If you wish to hide the natural colour differences of the wood you'll need something more covering than just Oil Plus 2C. You can use a pre-treatment like Precolor Easy which you finish off with Oil Plus 2C. However, if you have already treated the surface with Oil Plus 2C, you'll first have to remove the oil by sanding.
Tip: be careful with dark colours on blotchy wood species (e.g. maple, birch, ...). Dark colours will highlight the different patches in the wood. Use a light colour instead or use WoodPrep after sanding. This will provide a more consistent colour penetration.