The Shulhan Aruch (Orah Haim 214) discusses the importance of saying each word of a Beracha, and the guidelines concerning one who mistakenly omits a word. The basic rules are as follows:
1) If the word "Baruch" is omitted, and one began the Beracha with the words "Ata Hashem Elokenu," then the Beracha is not valid and must be repeated.
2) If one omitted the word "Ata," then according to the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Jerusalem, 1870-1939), his Beracha is invalid. The Arizal (Rav Yishak Luria of Safed, 1534-1572) revealed many deep meanings underlying the word "Ata" in a Beracha, and therefore, the Kaf Ha’haim writes, omitting this word invalidates the blessing. Hacham Ovadia Yosef, however, in Halichot Olam (vol. 2), disagrees, noting that as important as these "Kavanot" (meanings) are, this does not suffice to invalidate the Beracha if the word "Ata" was omitted. Therefore, if one mistakenly omitted the word "Ata," the Beracha is nevertheless valid.
3) If one mistakenly recited the word "Hashem" instead of actually pronouncing G-d’s Name in the Beracha, then as long as he properly recited the next word – "Elo-henu," his Beracha is valid. If he recited the words "Hashem Elokenu," without actually pronouncing either Name of G-d, then the Beracha is invalid. But if he recited "Elo-henu," then even though he had mistakenly said "Hashem," his Beracha is valid, since he did mention one of the Names of G-d in the Beracha.
4) If one mistakenly omitted the word "Melech," then the Beracha is invalid.
5) The Shulhan Aruch writes that if a person omitted the word "Ha’olam" in the Beracha, and recited only "Baruch Ata Hashem Elokenu Melech," the Beracha is not valid. It does not suffice to refer to G-d simply as "Melech" ("King"), without saying, "Melech Ha’olam" – that He is King over the entire universe. Therefore, according to the Shulhan Aruch, if one mistakenly omitted the word "Ha’olam" from a Beracha, the Beracha is not valid. This is also the position of the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909). Hacham Ovadia, in Halichot Olam, disagrees, noting that the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) mentions the position of the Eben Ha’ozer that a Beracha is valid even if the word "Ha’olam" is omitted. This view, Hacham Ovadia writes, suffices to make this a situation of "Safek Berachot" – where one is unsure whether he is required to recite a Beracha, as according to the Eben Ha’ozer, the Beracha was valid and does not have to be repeated. We therefore apply the famous rule of "Safek Berachot Le’hakel" – that we do not recite a Beracha if it is uncertain whether it is required.
However, in his work Hazon Ovadia – Berachot (p. 307), Hacham Ovadia retracted this ruling, and upheld the position of the Shulhan Aruch and Ben Ish Hai. He explained that the view of the Eben Ha’ozer runs counter to the vast majority of early Halachic authorities, and it therefore is not sufficient to turn this situation into one of "Safek Berachot." Hence, if one recited a Beracha and omitted the word "Ha’olam," then his Beracha is invalid. He must recite "Baruch Shem Kebod Malchuto Le’olam Va’ed" and then repeat the Beracha. Hacham Ovadia adds, however, that if one omitted "Ha’olam" but he recited "Ha’melech" ("the King") instead of simply "Melech," then the Beracha is valid, since the word "Ha’melech" implies that G-d is the single King over the earth.
Summary: If one recited a Beracha and omitted either the word "Baruch," the word "Melech," or the word "Ha’olam," then the Beracha is invalid and must be repeated. If he mistakenly said "Hashem" instead of "Ado-nai," then as long as he recited "Elo-henu" properly, his Beracha is valid. Omitting the word "Ata" does not invalidate the Beracha.