Long sounds (vowels and consonants)
[…] a Roman wishing to differentiate vowels in his own language and having Aristophanes’ system as a model before him would surely have borrowed for his purpose the signs for long and short vowels, that is to say, the macron and breve, which, so far as we know, always had forms (¯ and ˘) that they have retained to the present day.
― Revilo P. Oliver (‘Apex and Sicilicus’, The American Journal of Philology, 87:2, pp. 129–170). P. 143.
One of the best aspects of Ørberg’s work, is how it differentiates vowel length throughout. As explained by @TKR, a macron (ɔ: the dash – ◌̄ – on top of the vowel) denotes a long vowel. The Romans didn’t consistently use their version of macrons, but we have numerous inscriptions demonstrating how it was used, perhaps best known of this the Rēs gestae dīvī Augustī. In fact, they used two symbols for demonstrating a doubling of the phoneme (that is: the sound which the letter represents): an apex (plural apicēs) and a sīcīlicus (plural sīcīlicī). Oliver writes:
The handbooks of epigraphy all assure us that two distinct diacritical marks are found in Roman inscriptions: a phonetic sign, the apex, shaped like an acute accent (´), which was placed above a vowel-letter to show that the vowel was long and not short; and an orthographic sign, the sicilicus, shaped like a small inverted c or a modern comma (’), which was placed above a consonant to show that the consonant should be doubled (e.g., s̓ = ss).
― Oliver 1966: 129.
We also see the apex in the shape of a diagonal bar. Generally, both this symbol and the apex would begin where the letter ended, and extend somewhat over the next one (op. cit. 144).
Now, Quintilian claims that it is completely useless to mark long vowels where there is no doubt whether it should or should be a long vowel – in other words: Whenever you encounter a word which is differentiated only by vowel length, such as malus (evil, bad) vs. mālus (apple), you would add the macron (or for the Romans: the apex). The numerous inscriptions, however, clearly show that only marking long vowels when needed, was not the common practice.
The use of the apex in inscriptions makes it obvious that, from the very first, the function of the apex was to show the correct pronunciation of words that, presumably, were sometimes mispronounced in current speech.
― Op. cit. 135.
I longa – long I
In addition to this, a long i was not shown with a symbol, but with the so-called I longa, ɔ: ‘long I’. In the below example, you can see numerous very thin, delicate lines above many of the vowels (apicēs), and several examples of the I longa:
Sīcīlicus – ‘the small sickle’
There are, however, enough examples to support the definition given in our handbooks by showing that the mark was indeed used to indicate double consonants […]. [–––] Inscriptions of the late Republic and early Augustan period do show SABEL̓IO for Sabellio, OS̓A for ossa, MVM̓IAES as the ‘archaic’ genitive of Mummia, and possibly VET̓IVS for Vetius.
― Op. cit.: 145.
It is also worth noting that in mediaeval manuscripts, you very often will find a macron over consonants to show doubling or to show a missing nasal: damnū = damnum; gram̄aticus = grammaticus.
It is common for many dictionaries, such as the standard Lewis & Short dictionary (just google ‘Perseus Lewis and Short’) where you will find another symbol: the breve (◌̆), ɔ: the short vowel symbol. However, to continue from my opening quote:
But although a sign for a short vowel would have been decidedly useful in Latin for purposes of differentiation (so long as all long vowels were not marked), there is no indication that such a sign was ever used or even suggested.
― Op. cit.: 143.
That doesn’t mean we can’t find good use for it today. I find it very useful to use this symbol when I do my readings in LLPSI or other books, when memorising words which I tend to make mistakes with regards to vowel length. For example, is ‘river’ in Latin fluvius or flūvius? The latter is incorrect, so when practicing, I will write flŭvius to assure I remember the correct vowel length.