Complex sentence: It was also in that time that mass killings and arrests of Oromos began as a government attempt to curb the OLF's growth, which only served to further the OLF's support amongst Oromos.
Split into simpler sentences: It was also in this time that mass killings and arrests of Oromos began as a government attempt to curb the OLF's growth. This only served to further the OLF's support amongst Oromos.