A rotated rectangle consists of four points, the corners. Therefore it should be possible to convert it to a contour.
Then I would suggest you follow the answer on StackOverflow, linked below, to see if your point is within that contour.
Determine if a point is inside or outside of a shape with opencv
That would make you end up with something like this:
bool DoesRectangleContainPoint(RotatedRect rectangle, Point2f point) {
//Get the corner points.
Point2f corners[4];
rectangle.points(corners);
//Convert the point array to a vector.
//https://stackoverflow.com/a/8777619/1997617
Point2f* lastItemPointer = (corners + sizeof corners / sizeof corners[0]);
vector<Point2f> contour(corners, lastItemPointer);
//Check if the point is within the rectangle.
double indicator = pointPolygonTest(contour, point, false);
bool rectangleContainsPoint = (indicator >= 0);
return rectangleContainsPoint;
}
You could convert the rect into two triangles and then check to see if the point is inside the triangles. http://www.blackpawn.com/texts/pointinpoly/