The installation of wave energy converters (WECs) along the facing-wave side of in-development or pre-existing offshore structures provides an effective cost-sharing solution. In this study, a time-domain numerical model based on the modal expansion theory and the nonlinear potential flow theory, is applied to optimize the size and layout of an in-line array of oscillating water column (OWCs) and oscillating flaps (OFs) deployed along the facing-wave side of a very large floating structure (VLFS). The front/back-wall end of each OWC chamber is hinged by an OF device, and thus the number of OWCs is identical with that of OFs. Comparison with a typical OWC-VLFS integration shows that the addition of OFs can significantly reduce the vertical hydroelastic response of the VLFS. Additionally, wave energy extraction from the design of OFs hinged at the back-wall end of OWCs is enhanced due to more wave energy gathered in the chamber. Under the premise of the same space, the array layout demonstrates both higher conversion efficiency and hydroelastic reduction. The energy conversion is generally enhanced by increasing the gap, the OWC width, the OF height. The maximum efficiency increases firstly with wave height for weak wave nonlinearity and then decreases for strong wave nonlinearity.