Isn't there theory about a companion star in the Oort Cloud?
Yes.
But it asserts that the star comes close to Earth, like between Jupiter and Mars close, and that isn't possible.
I'd assume it has a near constant orbit, if it is elliptical it'd only come possibly as close as the outer edge of the Kuiper belt.
The theory is based off the periodic bombardment of the inner solar system byy waves of debris.