It appears that being an NGO participant at hearings requires little in relationship to the OSCE. This is unlike NGO Associate requirements in regards to the UN
This is from a report on the OSCE website
NGOs have played a vital role in the OSCE process since its very beginning. The OSCE participating States recognize that NGOs have valuable roles to fulfil in resolving conflicts, promoting tolerance, reporting on human rights issues, delivering humanitarian aid, conflict prevention and early warning. The OSCE has a policy of openness towards NGOs: it has no definition of a non-governmental organization and applies no criteria in its activities and relationship with NGOs other than that they shall not support terrorist activities. The OSCE works with relevant NGOs whether registered or not. The ODIHR has been asked by a participating State to assist it in the drafting of a new law on associations.
http://www.osce.org/odihr/16645?download=true
From reading the whole report I gather that NGO status is generally obtained from the entities own Government and they must meet their requirements.