This study evaluates the variability of organic aerosol (OA) sources monitored at the National Atmospheric Observatory Košetice (NAOK), a rural background site. Non-refractive PM1 was evaluated in four seasons of 2019. The positive matrix factorization with multi-linear engine was used to determine the sources of OA at NAOK, with five or four factors resolved for individual seasons. The probable origin areas of OA factors were identified by the ZeFir program. NAOK is affected by local sources of OA, as well as by regional and long-range transport of OA.