Results. We discovered 24 new compact sources, ranging in mass from 0.1 to 2.3, in size from 400 to 1300 au, and in density from 2.6 x 10(7) to 2.8 x 10(6) cm(-3). The masses of these sources are similar to those of the SMA protostars in OMC 3, but their typical sizes and densities are lower by a factor of ten. Only 8% of the new sources have infrared counterparts, but there are five associated CO molecular outflows. These sources are thus likely in the Class 0 evolutionary phase but it cannot be excluded that some of the sources might still be pre-stellar cores. The spatial analysis of the protostars shows that they are divided into small groups that coincide with previously identified jCMT/SCUBA 850 mu m and VLA NH3 chunps, which are separated by a quasi-equidistant length of approximate to 30' (0.06 pc). This separation is dominated by the Jeans length and therefore indicates that the main physical process in the filament evolution was thermal fragmentation. Within the protostellar groups, the typical separation is approximate to 6 '' (similar to 2500 au), which is a factor 2-3 smaller than the jeans length of the parental clumps within which the protostars are embedded. These results point to a hierarchical (two-level) thermal fragmentation process of the OMC In filament.