Open set domain adaptation (OSDA) aims to tackle the distribution shift of partially shared categories between the source and target domains, meanwhile identifying target samples non-appeared in source domain. The key issue behind this problem is to classify these various unseen samples as unknown category with the absent of relevant knowledge from the source domain. Though impressing performance, existing works neglect the complex semantic information and huge intra-category variation of unknown category, incapable of representing the complicated distribution. To overcome this, we propose a novel Unknown-Oriented Learning (UOL) framework for OSDA, and it is composed of three stages: true unknown excavation, false unknown suppression and known alignment. Specifically, to excavate the diverse semantic information in unknown category, the multi-unknown detector (MUD) equipped with weight discrepancy constraint is proposed in true unknown excavation. During false unknown suppression, Source-to-Target grAdient Graph (S2TAG) is constructed to select reliable target samples with the proposed super confidence criteria. Then, Target-to-Target grAdient Graph (T2TAG) exploits the geometric structure in gradient manifold to obtain confident pseudo labels for target data. At the last stage, known alignment, the known samples in the target domain are aligned with the source domain to alleviate the domain gap. Extensive experiments demonstrate the superiority of our method compared with state-of-the-art methods on three benchmarks.