Context.The THz atmospheric “windows”, centered at roughly 1.3 and 1.5 THz, contain numerous spectral lines of astronomical importance, including three high-J CO lines, the N II line at 205 μm, and the ground transition of para-H2D+. The CO lines are tracers of hot (several 100 K), dense gas; N II is a cooling line of diffuse, ionized gas; the H2D+ line is a non-depleting tracer of cold (~20 K), dense gas. Aims.As the THz lines benefit the study of diverse phenomena (from high-mass star-forming regions to the WIM to cold prestellar cores), we have built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the THz windows by ground-based observations. Methods.CONDOR was designed to be used at the Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test observations were made to characterize the instrument. Results.The combination of CONDOR on APEX successfully detected THz radiation from astronomical sources. CONDOR operated with typical $T_{\rm rec}=1600$ K and spectral Allan variance times of ~30 s. CONDOR's “first light” observations of CO 13-12 emission from the hot core Orion FIR 4 (= OMC1 South) revealed a narrow line with $T_{\rm MB}\approx 210$ K and $\Delta V\approx 5.4$ km s-1. A search for N II emission from the ionization front of the Orion Bar resulted in a non-detection. Conclusions.The successful deployment of CONDOR at APEX demonstrates the potential for making observations at THz frequencies from ground-based facilities.