On Monday, Orban's cabinet minister Antal Rogan said the government would further tighten restrictions on non-governmental organizations in its planned "Stop Soros" bill.
As well as bemoaning "burdensome" transparency requirements on funding from overseas, the group registered its particular anger over measures in the package invoking national security interests, such as rules which would require NGOs working with asylum seekers to be licensed by the government.
The OSF said it would "pursue all available legal avenues to defend the fundamental rights that are threatened by the legislation" and "continue to support the important work of civil society groups in Hungary".
Earlier, the Hungarian government had expressed their opposition to the activities of billionaire George Soros and his organizations, accusing them of meddling in the country's internal affairs.
Orban and Soros have clashed over the 2015 European migration crisis.
"The so-called Stop Soros package of laws is only the latest in a series of such attempts".
A major foundation funded by liberal philanthropist George Soros is closing its regional headquarters in Hungary, due to what it calls "an increasingly repressive political and legal environment" under prime minister Viktor Orban.
George Soros, founder and chairman of the Open Society Foundation, waits for the start of a meeting at European Union headquarters in Brussels on April 27, 2017.
The OSF said the campaign had "invoked anti-Semitic imagery from World War Two", a charge denied by the government.
The new legislation restricts the work of NGOs that receive foreign funding, imposing a punitive 25% overhead tax on donations.
The Central European University, founded in Budapest by Soros in 1991 after the fall of communism in eastern Europe, said on Tuesday it would stay in the Hungarian capital despite the OSF decision to leave.
"CEU can not go into another academic year in a situation of legal uncertainty", he said.