Osiris is the source of Dionysus mythology.
It's not innaccurate to say they are the same. One is Egyptian the other Greco-Egyptian.
The problem with studying any source, especially ancient, is that it requires more than taking things at face value. There are worldviews and perceptions that differ, sometimes radically, between ancient and modern thought.
The difficulty you're having with the others here and in your interpretation of Plutarch seems due to you either forgetting or not being familiar with two dominant and unavoidable influences of the ancient world: Hellenization and
interpretatio graeca. Hellenization is Greece's wide spread influence on other cultures. Egypt was greatly Hellenized thanks to the Ptolomies' 300 year dynasty, so there was an acceptance that helped to blur distinctions. Interpretatio graeca is the ancient Greek's version of comparative mythology, Hellenic myths and deities were the measure by which others' deities and myths were interpreted. The gods and myths of other cultures were that other culture's (less than perfect) understanding of deities.
When citing ancient sources, it's also important to recognize that quite often what is being cited is
a translation of that source for which the wording and nuances of those quotes can differ from one translation to the next. Scholars fluent in ancient Greek dialects can and do differ on how their interpretations are phrased. "Equated with" or "counterpart to" are closer to the mark than "same as" which implies identical - clones so to speak. Commonly it amounts to there being several parallels between god A and B.
As
Walter F. Otto notes in "Dionysus: Myth and Cult":
“The story of his [Dionysos] horrible death, which is mirrored in so many cult practices and is eloquently represented in one myth we still have, is undeniably like the famous myth of Osiris… This has let scholars even very recently to believe that the myth of Dionysus must be explained as a copy of the Egyptian myth… [and] the comparison of Dionysus with Osiris… constitutes a major portion of Plutarch’s essay on Isis and Osiris
However, the myth of the death of Osiris differs from that of Dionysus in far too many important points. Isis plays a significant role in the former, but the Dionysiac myth knows nothing of a figure comparable to her. Osiris is first shut up in a coffin and is killed in this way. Later Typhon tears him into fourteen parts which he scatters far and wide. Nothing is said about eating the dismembered body. And finally, Osiris meets his deadly fate when he has reached a man’s estate, after he has ruled for many years filled with blessings, while Dionysus is a boy when he is overcome by the Titans. These are not incidental characteristics. On the contrary, they give the death myth of Dionysus its character and make the similarities with the Osiris myth appear unessential.” - p. 195
A helpful book on the topic of interpretatio graeca is
Greco-Egyptian Interactions: Literature, Translation, and Culture, 500 BC–AD 300 by Ian Rutherford.
Another problem with assuming "same god, different name" is that there is not one simple history or definition of a god. Characteristics for one deity vary not only over time but could differ regionally, even within the same era. "Same god, different name" requires selectively choosing to observe just those elements that are similar and disregarding the entirety of a god's nature.
Even etymologically there is a connection. I forget exactly how but I have seen it broken down.
Their etymologies are unrelated. Dionysus descends from "dios" which is associated with Zeus. Osiris' name is NOT "Osiris" but Wesir. "Osiris" is actually the Latin transliteration of Wesir.
