Ottoman Turkish (; Turkish: Osmanlı Türkçesi), or the Ottoman language (Ottoman Turkish: لسان عثمانى, lisân-ı Osmânî, also known as تركجه, Türkçe or تركی, Türkî, "Turkish"; Turkish: Osmanlıca), is the variety of the Turkish language that was used in the Ottoman Empire. It borrows, in all aspects, extensively from Arabic and Persian, and it was written in the Ottoman Turkish alphabet. During the peak of Ottoman power, Persian and Arabic vocabulary accounted for up to 88% of its vocabulary, while words of Arabic origins heavily outnumbered native Turkish words.
Consequently, Ottoman Turkish was largely unintelligible to the less-educated lower-class and rural Turks, who continued to use kaba Türkçe ("raw/vulgar Turkish", as in Vulgar Latin), which used far fewer foreign loanwords and is the basis of the modern Turkish language. The Tanzimât era saw the application of the term "Ottoman" when referring to the language (لسان عثمانی lisân-ı Osmânî or عثمانليجه Osmanlıca) and the same distinction is made in Modern Turkish (Osmanlıca and Osmanlı Türkçesi).
Genitive case: answers the question كمڭ kimiñ ("whose?"), formed with the suffix ڭ –ıñ, –iñ, –uñ, –üñ. E.g. پاشانڭ paşanıñ ("the pasha's") from پاشا paşa ("pasha").
Accusative case (definite): answers the question كمى kimi ("whom?") and نه يى neyi ("what?"), formed with the suffix ى –ı, -i: طاوشانى گترمش ṭavşanı getürmiş ("he/she brought the rabbit"). The variant suffix –u, –ü does not occur in Ottoman Turkish unlike in Modern Turkish because of the lack of labial vowel harmony. Thus, كولى göli ("the lake".ACC), but Modern Turkish has gölü.
Locative case: answers the question نره ده nerede ("where?"), formed with the suffix ده –de, –da: مكتبده mektebde ("at school"), قفصده ḳafeṣde ("in a cage"), باشده başda ("at the start"), شهرده şehirde ("in town"). As with the indefinite accusative case, the variant suffix –te, –ta does not occur unlike in Modern Turkish.
Ablative case: answers the questions نره دن nereden ("from where?") and ندن neden ("why?").
The conjugation for the aorist tense is as follows:
Ottoman Turkish was highly influenced by Arabic and Persian. Arabic and Persian words in the language amounted for up to 88% of its vocabulary. As in most other Turkic and other foreign languages of Islamic communities, the Arabic borrowings were not originally the result of a direct exposure of Ottoman Turkish to Arabic, a fact that is evidenced by the typically Persian phonological mutation of the words of Arabic origin.  
The conservation of archaic phonological features of the Arabic borrowings furthermore suggests that Arabic-incorporated Persian was absorbed into pre-Ottoman Turkic at an early stage, when the speakers were still located to the north-east of Persia, prior to the westward migration of the Islamic Turkic tribes. An additional argument for this is that Ottoman Turkish shares the Persian character of its Arabic borrowings with other Turkic languages that had even less interaction with Arabic, such as Tatar and Uyghur. From the early ages of the Ottoman Empire, borrowings from Arabic and Persian were so abundant that original Turkish words were hard to find. In Ottoman, one may find whole passages in Arabic and Persian incorporated into the text. It was however not only extensive loaning of words, but along with them much of the grammatical systems of Persian and Arabic.
In a social and pragmatic sense, there were (at least) three variants of Ottoman Turkish:
Fasih Türkçe (Eloquent Turkish): the language of poetry and administration, Ottoman Turkish in its strict sense;
Orta Türkçe (Middle Turkish): the language of higher classes and trade;
Kaba Türkçe (Rough Turkish): the language of lower classes.
A person would use each of the varieties above for different purposes, with the fasih variant being the most heavily suffused with Arabic and Persian words and kaba the least. For example, a scribe would use the Arabic asel (عسل) to refer to honey when writing a document but would use the native Turkish word bal when buying it.
Historically, Ottoman Turkish was transformed in three eras:
Eski Osmanlı Türkçesi (Old Ottoman Turkish): the version of Ottoman Turkish used until the 16th century. It was almost identical with the Turkish used by Seljuk empire and Anatolian beyliks and was often regarded as part of Eski Anadolu Türkçesi (Old Anatolian Turkish).
Orta Osmanlı Türkçesi (Middle Ottoman Turkish) or Klasik Osmanlıca (Classical Ottoman Turkish): the language of poetry and administration from the 16th century until Tanzimat. It is the version of Ottoman Turkish that comes to most people's minds.
Yeni Osmanlı Türkçesi (New Ottoman Turkish): the version shaped from the 1850s to the 20th century under the influence of journalism and Western-oriented literature.
Historically speaking, Ottoman Turkish is not the predecessor of modern Turkish. Rather, the standard Turkish of today is essentially Türkiye Türkçesi (Turkish of Turkey) as written in the Latin alphabet and with an abundance of neologisms added, which means there are now far fewer loan words from other languages. However, Ottoman was not instantly transformed into the Turkish of today. At first, it was only the script that was changed, and while some households continued to use the Arabic system in private, most of the Turkish population was illiterate at the time, making the switch to the Latin alphabet much easier. Then, loan words were taken out, and new words fitting the growing amount of technology were introduced. Until the 1960s, Ottoman Turkish was at least partially intelligible with the Turkish of that day. One major difference between modern Turkish and Ottoman Turkish is the former's abandonment of compound word formation according to Arabic and Persian grammar rules. The usage of such phrases still exists in modern Turkish but only to a very limited extent and usually in specialist contexts; for example, the Persian genitive construction takdîr-i ilâhî (which reads literally as "the preordaining of the divine" and translates as "divine dispensation" or "destiny") is used, as opposed to the normative modern Turkish construction, ilâhî takdîr (literally, "divine preordaining").
Calendar in Thessaloniki 1896, a cosmopolitan city; the first three lines in Ottoman script
The transliteration system of the İslâm Ansiklopedisi has become a de facto standard in Oriental studies for the transliteration of Ottoman Turkish texts. Concerning transcription the New Redhouse, Karl Steuerwald and Ferit Develioğlu dictionaries have become standard. Another transliteration system is the Deutsche Morgenländische Gesellschaft (DMG), which provides a transliteration system for any Turkic language written in Arabic script. There are not many differences between the İA and the DMG transliteration systems.
M. Sukru Hanioglu, “A Brief History of the Late Ottoman Empire”, Published by Princeton University Press, 2008. p. 34: “It employed a predominant Turkish syntax, but was heavily influenced by Persian and (initially through Persian) Arabic.
Pierre A. MacKay, "The Fountain at Hadji Mustapha," Hesperia, Vol. 36, No. 2 (Apr. - Jun., 1967), pp. 193-195: "The immense Arabic contribution to the lexicon of Ottoman Turkish came rather through Persian than directly, and the sound of Arabic words in Persian syntax would be far more familiar to a Turkish ear than correct Arabic".