Oppression of non-ethnic Hungarians in Hungary pre 1800s

There are many books about Magyarization of Slavic and Romanian ethnic dwellers. About the Magyarization of Romanians it is a splendid book, in Romanian:
G. Popa Lisseanu - Date privitoare la maghiarizarea romanilor, Carte veche, Editura: Universul, 1937.

There was no really such pre 1800. As Samuel Russel points out, the modern concept of nations was born only during and after the French revolution. Hungary and Transylvania from the late medieval times were part of the Habsburg empire, but they were not united until 1867. The official languages of Hungary were the following:

1000-1784: Latin

1784-1836: German

1836-1844: German and Hungarian

1844-1849: Hungarian

1849-1867: German

1867-1918: Hungarian

This shows that there could not be a Magyarization process because even Hungarian was oppressed before 1836.

Hungary had the same sort of autonomy within the empire as the other Habsburg principalities had (Transylvania, Galicia, etc.), but in principle it was an absolute monarchy.

Most of the noble families were Hungarian speaking, even though there were several areas of Hungary were the majority was non-Hungarian (like Upper Hungary which became later Slovakia or Transcarpatia which is now in Ukraine). There were also a few noble families of non-Hungarian origin, but they also learnt Hungarian to some extent. This means in particular that within the nobility the Hungarians were over-represented, but not in other areas of the society.

Moreover, in the 1830-40s one Hungarian course become compulsory in schools, and in the 1848 revolution the Hungarian leaders wanted to introduce Hungarian as the only language of the education (this did not happened in the end since the independence war was lost against the Habsburgs).

Magyarization usually refers to the oppressive politics during the dualist times in 1867-1918, when the Habsburg Empire was reorganized into a union of two member states, Austria and Hungary. This was also when Transylvania was united with Hungary.

While the constitution then gave individual rights to the people of the minorities, it did not give community rights to them, that is, an equal status to their language (except for Croatian in Hungary, and to some extent Czech in Austria). Hence, in the Austrian part there was Germanization while in the Hungarian part there was some Magyarization.

Even during these times the ratio of minorities increased at some places, although there was also some places where it decreased. There were also some notable exceptions, for example bank notes were printed in six languages, several state documents (like birth and marriage certificates) were available in minority languages as well, and church service and religious education were absolutely legal (and equally funded by the state) in minority languages.

Croatia had an extensive autonomy within Hungary, and Croatian was the only official language there. The oppression became stronger in 1907 though, from when the minority and religious schools were forced to teach mostly in Hungarian (by the Lex Apponyi) until the end of the First World War (while there was still a little allowed time for minority classes).

It seems that by "oppression" the OP means interdictions against the language, religion and other cultural aspects of the dominated peoples.

As Transylvania was a historical region of Hungary from the creation of the kingdom, and given the Hungarian kingdom, like many others, was created by invading warriors, some level of "oppression" against conquered peoples (of Slavic, Latin or other origin) must have been inevitable from the very beginning.

It is impossible to equate the result of conquest and political domination (by Hungarians in Transylvania) with the assimilationist politics of modern states which would require the modern infrastructure of a centralized education system following a nationalistic agenda. All those factors have been absent in Transylvania until the nineteenth century. But there are some clear events and aspects of cultural change, acculturation, expropriation, marginalization of Romanians/Vlachs before the nineteenth century in Transylvania.

General trends

This process is not surprising for any conquered people in the middle ages, and it may have two contradictory effects: of assimilation of the subordinated culture, when it is at least partially integrated or replaced, or of conservation, when it is marginalized and isolated, thus keeping its autonomy. Both effects can be seen in our case.

Two aspects are notable.

• First, the political organization in Transylvania prior to Hungarian conquest, as far as it is documented, must have been rather limited. If any, Romanian polities there must have been some type of "Popular Vallachies" with limited territories and even more limited centralization around nobility centers. Cities and castles were practically absent. Thus, there must have been little organizational Romanian resistance and autonomy at political level (compared to other regions that became part of Hungary, like Croatia or Slovakia), but these were not absent; a region that kept longer its political autonomy and thus is better documented is that of Maramures, which is the region of origin of both Dragos, the first prince of Moldavia, and of Bogdan, its first independent ruler.

• Second, the Romanian population was not Catholic, but Christian-Orthodox. Romanian nobility, as far as it was present, must have felt a strong pressure from the very beginning to integrate the Hungarian nobility, which meant integration into the Catholic church and gradual adoption of Hungarian language. (There is some proof of this process. There are some known Hungarian nobles of Romanian, Slavic or Cuman origin, the most famous being maybe the Hunyadi, who became a royal family.) The ones that resisted this process or were excluded from it had an incentive to leave, and they played a clear role in the founding of Moldavia and a possible/probable role in the founding/unification of Wallachia. On the other hand, vastly outnumbering the nobles, the peasants had no incentive to follow a parallel trend. Lacking all political liberties at higher levels, they enjoyed a de facto cultural autonomy at the lower levels of organization, based on their religious identity, which was enforced by geographical isolation, especially in Maramures and the Apuseni Mountains. Some efforts of imposing the Catholic faith are documented, with no important success over the peasantry, but they could have played a role in some processes of migration from Transylvania to Moldavia and Wallachia.

These two aspects contributed to the two contradictory effects mentioned above: while Romanian elites gradually disappeared, the Orthodox faith of the commoners along with their political backwardness (conservative, local, rural organization) helped them keep their language and other aspects of ethnic identity.

Historical stages

• Before the fourteen century, Romanian communities had been represented up to some point at the higher levels of organization of Transylvania, as one of the four estates (Universitas Valachorum). In 1366 the Decree of Turda issued by king Louis I redefined nobility as membership in the Roman Catholic Church.

• Directed against the whole peasantry (Romanian and Hungarian), a capital event that confirmed the process described above and enforced its results is the Unio Trium Nationum accord of 1438 between the Hungarian nobility, the German city patricians and the free military Székelys.

• After the Ottoman conquest of main Hungary (1526), the principality of Transylvania enjoyed a large autonomy under Ottoman suzerainty, Lutheranism and Calvinism became very present and, while the Orthodox church was not treated as equal to other confessions, religious tolerance and plurality flourished, a hallmark of Transylvanian identity until modern times.

• After 1683 the Habsburg controlled Transylvania and started a process of counter-reformation, oriented mainly against the Calvinist faith of Hungarian nobility. Another result was the creation of the Romanian Greek-Catholic Church, the members of which could gain equal rights with those of the other Transylvanian nations. It was an important event in the history of Romanian modern culture, as it opened the way to the creation of an intelectual Romanian class in Transylvania that was previously limited to lower clergy.

Outcome

While not completely absent, given the aforementioned processes, a Hungarian cultural oppression against Romanians before the nineteenth century is more of a myth than a reality. Either this process was very weak or very ineffective. The fact that Romanians were kept at the fringes of nobiliar and urban Transylvania for a long time was not a peril to the conservation of the language before the modern era of mass education.

An illustrative fact is the ethnographical map of 1910 Hungary, with Romanians inhabiting the larger areas of Transylvania, which on the other hand was the less populated part of Hungary. (It looks like not too many Hungarians came to Transylvania even after the full reintegration into Hungary in 1867; the most compact populations are still those of the German cities and of the frontier Székelys. Hungary seems to have considered Transylvania in terms of historical appurtenance rather then territory and population numbers. That stands in stark contrast to Romanian policy in newly acquired territories like Dobrudja, which became much more populated.) Geographical and ethographical facts may help relativise the present question, as well as similar ones.