We have no date of birth for Ornendil, but according to The Heirs of Elendil in volume XII of The History of the Middle-earth, his younger brother Aldamir was born in III 1330. Ornendil must therefore have been born sometime in the period preceding that year.

Ornendil, incidentally, represents one of the few cases where The Heirs of Elendil differs from the canonical dating scheme. According to that source, he was slain in the year III 1446, while the canonical Tale of Years places his death in III 1437. Specifically, the Tale of Years says in its annal for III 1437: 'Eldacar flees to Rhovanion; his son Ornendil is murdered.' That annal certainly seems to date Ornendil's death as III 1437, though it might perhaps be giving an abbreviated account of the events of that year to include their immediate aftermath, so that 'his son Ornendil is murdered' might be read as 'some years later, his son Ornendil is murdered'. That is a somewhat awkward reading, but it would help to address the apparent disparity between sources.

Names ending in -ndil present a problem when it comes to pronunciation, and in particular the question of where stress should be placed within the name. If we treat -nd- as two separate sounds (as they are in English), then the whole name would be stressed on the second syllable ('orne'ndil'). In Elvish, however -nd- is represented by a single character, which would place the stress on the first syllable ('o'rnendil', as shown above). A reasonable argument could be made for either pattern of stress within the name.

King Eldacar gave both his sons names tied to trees (Ornendil's younger brother Aldamir had a name meaning 'tree jewel'). It should be noted that the word 'friend' given here does not quite convey the sense of devotion implied by the Elvish ending -ndil. These 'tree' names may have been meant to connect the sons of Eldacar with the White Tree of Minas Tirith, or they may have been intended to show devotion to trees in a more general sense.

Ornendil's sister is mentioned obliquely in The Heirs of Elendil in volume XII of The History of Middle-earth. Our only definite information about her is that she was older than Ornendil's brother Aldamir, so in principle she may also have been older than Ornendil himself.