Most dogs do not handle oral oleander well

Although canine trials with oleander were quite successful, it was injected oleander medicine and not oral.  Dogs digestive systems just don't seem to handle oleander well at all so much so that we had to drop oral oleander as a recommendation for dogs.

What you might try is getting the liquid oleander extract (or making your own from the instructions on my site) and using a rectal syringe.  That will largely bypass the digestive system and get the oleander to the problem area.

You might find this useful:

An Anti-Cancer Protocol for Dogs