Besides the original Ogham fews, there are several sets of additional ones listed in “The Scholars’ Primer (Auraicept na n-Eces)”. Called the Forfeda, these additional letters include diphthongs and abbreviations. Except for the diphthongs, the other Forfeda do not seem to be translated or readily used.
According to the materials from the Grey School of Wizardry class, “Tree Oghams and Divination,” “After Greek language came to Ireland, the people using the Ogham…needed other symbols for the diphthongs and extra sounds in Greek and so the Forfeda were added, EA-AE.” Rev. Robert Ellison, author of this class, adds that though there are four sets of Forfeda, only one set was used with any regularity, and found on Ogham stones. He says that this set was “added for the new sounds coming into the Irish language.” I believe that this particular set was the diphthongs (EA, OI, UI, IO, and AE).
Ellison adds in his book, “Ogham: The Secret Language of the Druids,” that the lists of abbreviations and additional letters (Forfeda) “would fall in with the use of the Oghams as a regular alphabet (Page 61).” However many were not translated, except for the set of diphthongs, which have been integrated in the Cipher Ogham and numbering lists. Moreover, one of the most well-known of the various Oghams, Fionn’s Window (#103), features the diphthongs on its second round circle between each of the second fews of the four original aicme. The Oghams that include these letters are The Three Ridge Ogham (#31), Unnamed Ogham (#34), Uproar of Anger Ogham (#63), Fraudulent Ogham (#87), and the Strand of Ferchertne (#103).
Supporting my idea that the diphthongs were regularly used is that they also have kennings as do the original fews. The fifth aicme, first few – “Eabhadh (White Popular): EA” had the most, since it was used often. The second through fifth fews are referenced in the Word Ogham of Morann MacMain as well as the Word Ogham of Mac ind Oic.
Ellison, Robert, “Ogham: The Secret Language of the Druids,” ADF, 2007.
----, “Tree Ogham and Divination,” Grey School of Wizardry, class.