Please read the ANSYS Help description of the algorithm used in Fluid Pressure.
Fluid pressure along normal and tangential directions can penetrate into the contact interface from one or multiple locations. The fluid pressure-penetration load has a path-dependent nature. The penetrating path can propagate and vary, and it is determined iteratively. At the beginning of each iteration, the program first detects starting points which are exposed to the fluid pressure. Among the starting points, the program then finds fluid penetrating points where the contact status is open or lost, or where the contact pressure is smaller than the user-defined pressure-penetration criterion. When a contact detection point has a contact condition of “penetrating,” it is subjected to the fluid pressure, and its nearest neighboring nodes are considered to be the starting points which are exposed to the fluid pressure as well.
The fluid pressure is not applied to an area having a contact status of open unless the edges/ends of the area belong to the starting points.
After reading the above, it seems that choosing a point on the bottom of the O-ring is a better choice.