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I succeeded!

This preamble works:
\documentclass[a4paper,12pt,dutch]{article}
\usepackage[utf8]{inputenc} %% x
\usepackage[T1]{fontenc}
\usepackage[dutch]{babel}
\usepackage[default,oldstyle,scale=0.95]{opensans}
\usepackage{lmodern}

Following had to be disabled, since it loads CM-sans ...

I followed your advice, but still no luck.

I used a virtual machine and did an installation of TEX-live and texlive-fonts-extra package. But all I see in the PDF-file is CM and other (unrecognised) font. Opensans is absent!
I don't receive any error or warning messages.

Bartman: what do you mean ...

Hallo LaTeX-community,

I tried to install opensans.tds.zip (CTAN) by copying the directories of the zip-file into my /usr/share/texmf (Yes, Linux user here). After that I run ' mktexlsr ' and ' updmap-user --enable Map opensans.map ' (as root).
In my LaTeX document, I said: \usepackage{opensans ...