Getting oil residue out of a container
You can pour water and detergent inside, let it sit that way for a day to soften, then clean it by shaking the detergent solution vigorously and then filling it with water and emptying until the water comes out non-soapy. This should work, unless you let it sit for a long time empty, but with the residue of polyunsaturated oils inside. In that case, they can have gummed up. 

If the brush problem comes because a straight brush can be stuck inside, but can't reach the nooks and the bottom, you could use a curved brush. These are sold to homebrewers in large sizes, I don't know if they are available for single bottles too.