Orabel is a medieval Scottish variant of Arabella, from the Latin orabilis, meaning "prayerful; invokeable." Other variants include Orabilis, Orabilia, Orabella, Arabelle, Orbella and Arbelle, but it has seen a handful of other spelling options. Some of those spellings, such as Orbel, became surnames. Ora is another form, simply from oro, meaning "to pray." As Orabel, an Anglicized form of Orabilia, the name was used more commonly in the 13th century and on. Arabella, also Scottish in origin, didn't become as well known until the 16th century.
Ora could easily be a nickname, but as a full name it has some namesakes: Ora Alexander, a blues singer; Ora Carew, a silent film actress; Ora Washington, a tennis player; and most recently Rita Ora, a British singer. This name was used in the early 1900's in America and last ranked in 1962. Ora was also a Balto-Slavic (Albanian) goddess or spiritual guardian.
One of the earliest/only uses of Orabel was in the 12th century Chanson de Guillaume, in which the name is spelled Orable and used for a princess of Saracen. (I suspect her name might be more about the river Orbieu than the Scottish name Orabel.) In the story the main character Guillaume marries her. Most recently, Orabelle has been given to a Belgian-style ale.
Orabel, any way you spell it, is an exceedingly rare name today with no babies given the name in recent years. Arabelle, however, is up to 111 births in 2013, and Arabella at 1,512 births in 2013, ranking at #210.