It's always correct to put a comma before "and" where ambiguity would otherwise ensue:

1. I want to thank my parents, Sam and Dave.

***> Unless you are indeed the result of some curious mono-sexual coupling, change to:

        1a. I want to thank my parents, Sam, and Dave.

                                    Elsewhere, there are two schools of thought.

School A says that where the last term of a list is introduced by "and", it is redundant to put a comma before that "and".

Thus School A prefers:

2. I bought some apples, pears and oranges.

  ***> School B on the other hand thinks that the comma before "and" denotes a pause, and is therefore not redundant.

            Thus School B prefers:

             2a. I bought some apples, pears, and oranges.

                       (For some reason, lists of fruits are obligatory in this particular debate.)

In British English, School A is predominant, especially in newspapers and magazines.

However, some academic publishers (notably Oxford University Press; whence this comma's name, the "Oxford comma") belong to School B.

[Using English.com]

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