What you should do is reformatting the sections, subsections, &c. Redefining numbering as you do changes the section numbers are printed everywhere in the document (incl. title of contents and referencing in the text!).
Here is a way, using the titlesec package. As OpenSans is not installed on my system, I repplaced it with
cabin – another sansserif font.
In this code, in short the first argument of \titleformat is the name of the sectioning command, the second (between brackets) is its style . The third argument is made of commands that are applied to both the label and the title, the fourth formats the label, the fifth is the spacing between label and title and in the sixth are commands that are applied before the title; with option
explicit, the title can be referred to as #1. There is a seventh (optiaanl) argument, unused here, that contains code to be applied after the title.
The spacing command has the same first argument; the second is the increase of the left margin, the third is vertical space before the title and the fourth is the spacing between title and the following text.
The interpretation of the different parameters can vary according to the style of the sectioning command, and mamy more details are explained in the documentation, § 3.