possessive form of nouns ending in "x"

Where a noun ends with the letter x, is it proper for the possessive form of the word to end with 's or simply ' ?

Example 1:

"It is Xerox' position that it owns the patent."

vs.

"It is Xerox's position that it owns the patent."

Example 2:

"The ox's tail was short."

vs.

"The ox' tail was short."