Does avocado and orange have a common ancestor?
Here is a website that presents very accuerately the tree of life: tolweb.org/tree 

Yes, they have a common ancestor just like any other living things! How closely related are they?

Both species are:


Eukaryotes (cells with a nucleus)
Archaeplastidae (plants)
Angiosperms (flowering plants)


Then, they split their way! Here is the tolweb.org page that correspond to this speciation: http://tolweb.org/Angiosperms/20646

The avocado is in the Laurales family: http://tolweb.org/Laurales/20672

The orange is in the Rutacea (sapindales, rosids, eudicots) family: http://tolweb.org/Sapindales/21051.

You can use the left arrow on the tree of these pages to come back through times and see the different speciation event that seperate both species.

While the orange is a eudicots, the avocado is part of the magnolids. The oldest fossil records of an angiosperm lived about 132MYA (see Crane et al., 2004). So the two species diverge for quite a long time! In order to give a comparison, they are as distant than humans and rabbits (it is a rough personal estimation, I don't have any reference for that!)

Note: The orange is called Citrus sinensis in latin. The species originated in China. The avocado is called Persea americana and it originated in central american (as their names indicate).

I don't know what make you feel that they are closely related. Maybe someone might say some words about their chemical compounds (and their flavour) but at first glance these two fruits taste very differently to me. Or maybe they have some remarkably similar phenotypic traits due to convergence evolution... I don't quite know about that!