We present the measurement of entanglement between twin beams generated with a doubly resonant optical parameter oscillator (OPO) based on four-wave mixing in hot 85 Rb vapor above threshold. This is the first measurement of entanglement in an OPO with χ (3) media above threshold. We reconstruct the covariance matrix for several configurations and with a full picture of the four side band mode state, we study entanglement between all possible bi-partitions. We show a robust generation of entanglement with stronger generation for a specific pair of modes. For this system, we show that atomic density is a determinant factor for generation and loss of quantum correlations. The generation of entangled fields by an atomic OPO close to atomic resonance of alkali atoms enables natural integration into quantum networks.