The Defence Assistance Organisation (Bulgarian: Организация за съдействие на отбраната, ОСО), initially the Voluntary Defence Assistance Organisation (Bulgarian: Доброволна организация за съдействие на отбраната, ДОСО) was a government-sponsored network of youth clubs in the People's Republic of Bulgaria.

It primarily consisted of various interest clubs for practical education in driving and automotive mechanics, amateur radio, sailing, shooting, paragliding, flight training and airmanship, diving and many others.

DAO clubs were part of a concentric nationwide network and were usually attached to factories, collective farms and schools.

In exchange for a symbolic membership fee, DAO members could access all types of training for free.

It was re-established in 1977 as the Organisation for Military Technical Preparation of the Population, and again reorganised as DAO under the Council of Ministers in 1982.