From David Waltz, "The LDS Quad Metaphysics and the Early Creeds" (copy in my possession):
Athanasius speaking of the Father and the Son as being of the "same substance" being used as a generic sense
Put simply this means that just as all men share in one nature (humanity), so too God the Father and His only-begotten Son share in one nature (divinity).
"One God" in LDS Scripture:
[some] affirm that there is a concrete sense in which three persons, Father, Son and Holy Ghost, are/is “one God”.
Re. other texts (e.g., Moses 1:20) and how to harmonize the senses of "One God":
I think it is quite clear now that the LDS quad has two distinct senses in which the phrase “one God” is used. A hasty reflection by some will yield a conclusion that the two senses are contradictory. However, as indicated, this is, IMO a hasty response, for I believe that there exists a simple solution/reconciliation of the two senses.
The harmonization of these two senses is quite simple if one understands the noun “God” as being used in two distinct forms; first as a descriptive predicate noun (in essence adjectival); and second as an identifying predicate noun (i.e. titicular).
In the first sense/form God is being used as a synonym of divinity (divinity = the quality of being divine/godlike). This is the sense/form in which the Father, Son and Holy Ghost are called “one God”. By illustration all humans can be called “one Man”, in that we all share in one humanity (humanity = the quality of being human).
In the second sense/form (God as a title), the referent, when used with the definite article (the), is almost always God the Father. In my estimation there exists a very good reason for this: God the Father is the source, or fount of divinity (Latin: fons totius divinitatis); the divinity of the Son and the Holy Ghost flows from the Father. As such, God the Father is “the one God”.