To make the pronunciation easier, it is possible to insert a vowel o between two roots in a composite: puŝoŝipo – tugboat, skribotablo – writing desk. With some words the letter o is inserted to make them more recognizable, because of the tradition or because the words have ends with an o in international usage: diosimila – like a god, radioelsendi – radiobroadcast.
The letter o cannot be inserted in front of suffixes or after prefixes.
In PAG^[33], this problem is described differently – as conserving of endings. The conserving of endings has the same reason as inserting of an o. However, in this case the o between two roots is not an inserted vowel but an nominal ending of the first root. Another consequence is, that also adjectival ending a and adverbial e can be found between two roots. The type of the ending is driven by so called vortefiko rules (vortefiko – effect of the word)^[34].
This theory seams reasonable to me. Except the problems with vortefiko rules, I do not know if the theory could be confronted with real data. This problem would require further study, especially of a large corpus. Provisionally, I look at the o as an inserted character.

According to PAG^[35], hyphen (dividstreko) in composites is used in following cases:

For other uses of a hyphen, see also chapters 3.3.1.1 Declination of proper names and 4.3.5 Abbreviations

Names of sciences are full of pseudosuffixes^[36], however some of the sciences can be regarded as composites.
The most often suffix is io (unofficial), that makes the name of the science from the scientist. The scientist ends very often with ologo – partly pseudosuffix (astrologo, ekologo), partly unofficial suffix (antrop|ologo, soci|ologo).
Words for sciences sometimes contain pseudosuffix iko (poetiko – poetics, stylistko – stylistics). Ik can be regarded in some cases as suffix forming the name for the science from the scientist (stylisto – stylist). However these cases are very rare and the rest before the iko is very often not a scientist (simbolo – symbol, simboliko – symbolism) or the result is not a science (gimnasto – gymnast, gimnastiko – gymnastics).

The problem of the names of countries and nationalities has been often discussed. There are two ways – to form the name of the inhabitant from its country or vice versa. The current state of the names of countries is evolved tradition, international influences and tendency to use some simple system.
Originally, an inhabitant was primary for the Old Word and a country for the New World; with some exceptions. The inhabitant was formed by the suffix ano and the country by suffix ujo. Names of some were derived from a town or a river by the suffix io(Meksiko – Ciudad de Mexico ◊ Meksikio – Mexico).
However, there was a tendency to make the names more international. Some names were using the word lando (Finnlando), the suffixio was used more and more instead of the suffix ujo and a new suffix istan was used for some countries.
Today, there is a list of standard names of countries (Listo de normaj landnomoj)^[37]. This list put all countries into two categories and some subcategories:
1) Country is primary, inhabitant is formed by suffix ano.
Peruo ◊ Peru|ano, Aŭstralio ◊ Aŭstrali|ano, Nepalo ◊ Nepal|ano
2) Inhabitant is primary, country is formed by various suffixes:
a) by the suffixes io or ujo.
Hungaro ◊ Hungario/Hungarujo, Turko ◊ Turkio/Turkujo
b) by the root lando.
Finno ◊ Finnlando, Skoto ◊ Skotlando
c) by the suffix istano.^[38]
Uzbeko ◊ Uzbekistano, Afgano ◊ Afganistano
The names derived from the name of a town or a river by the suffix io are in the first category.

Abbreviations (mallongigoj) have nearly the same form as in other languages:
E.g.: ekz. – ekzemle – for example, k.t.p. – kaj tiel plu – etc., p. – pago – page, t.e. – tio estas – i.e., PIV – Plena Ilustrita Vortaro – The Full Illustrated Dictionary
Very often, the abbreviations are formed by conserving few letters from the beginning and possibly some from the end and by replacing the rest by a hyphen. Such abbreviation has grammatical ending and is normally declined.
d-ro – doktoro – doctor, s-ro – sinjoro – Mister, s-rino – sinjorino – madam

^[33] See PAG §309
^[34] See chapter 4.1.1.
^[35] See PAG §14B – I.
^[36] Sequences of characters that very often repeat in Esperanto words, mostly suffixes in languages the words originate from. See chapter 4.2.5.

^[37] Oficialaj Informoj de la Akademio de Esperanto, n-ro 9, 1989

^[38] Pakistano belongs to the lexicon country, the inhabitant is called Pakistan|ano.