A typical surround-view system consists of four fisheye cameras. By performing an offline calibration that determines both the intrinsics and extrinsics of the system, surround-view images can be synthesized at runtime. However, poses of calibrated cameras sometimes may change. In such a case, if cameras' extrinsics are not updated accordingly, observable geometric misalignment will appear in surround-views. Most existing solutions to this problem resort to re-calibration, which is quite cumbersome. Thus, how to correct cameras' extrinsics in an online manner without using re-calibration is still an open issue. In this paper, we attempt to propose a novel solution to this problem and the proposed solution is referred to as 'Online Extrinsics Correction for the Surround-view system OECS for short. We first design a Bi-Camera error model, measuring the photometric discrepancy between two corresponding pixels on images captured by two adjacent cameras. Then, by minimizing the system's overall BiCamera error, cameras' extrinsics can be optimized and the optimization is conducted within a sparse direct framework. The efficacy and efficiency of OECS are validated by experiments. Data and source code used in this work are publicly available at https://z619850002.github.io/OECage/.