I think it's mostly just mental laziness producing familiair auditory patterns, rather like a parrot does.

Example:

Virtually every American says "harry carry" when referring to a well-known Japanese expression for suicide. The real expression is "hara kiri".

That's pronounced Harrah kee-ree, as should be obvious from the spelling "hara kiri". It's not harry carry! Why do Americans say "harry carry"? Well, it's easy to say because it rhymes in a singsong manner like "see ya later, alligator". To notice that it's actually hara kiri demands a little careful attention. To say harry carry does not. Thus the lazy and basically not very attentive mind just morphs hara kiri into harry carry without even noticing.

It's usually just a lazy, hurrying, not very attentive mind that juxtaposes "Osama" and "Obama", and it doesn't even notice while it's doing it. Why doesn't it notice? Well, probably because it's already fully occupied with the next thing it's about to say...