Dear @Riad.Elhamoud,
OpenFAST does not generally make use of small angle assumptions. However, a few modules such ElastoDyn do assume small angles (with correction for orthogonality that extend the validity up to about 15 degrees angle) for the blade deflection, tower deflection, and platform rotation. When a threshold is reached for the blade, tower, and platform rotations, OpenFAST will issue a warning. Other angles can exhibit large rotation and invoke fully treatment of nonlinearity without loss of accuracy.
A similar question was discussed in the following forum topic: Valid Yaw values for OpenFAST.
Best regards,