The mission follows an invitation from Turkish authorities, the OSCE's Office for Democratic Institutions and Human Rights told reporters in Ankara.
But Tana de Zulueta, head of the mission, admitted there were limitations on freedom of movement.
"We already have limitations in place on freedom of movements for international observers in some parts of the country. We have to take these into account," Zulueta said.
Zulueta did not give further details on the "limitations" but the Turkish military has been waging a relentless campaign in the southeast of the country against Kurdish militants.
A team of 11 experts in Ankara and 24 observers from 13 countries will be deployed across the country.
The Turkish public will vote on April 16 on whether to approve constitutional changes that would create an executive presidency.
While the government argues that the changes are necessary for political stability, critics fear it will lead to one-man rule and a further erosion of democracy.
The observers will meet with representatives from civil society, media, political parties and Turkish officials as well as visiting polling stations across the country.
But this will not constitute systematic observation, she added.
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