I'm not 100% sure as I haven't tried, but I think the problem you're seeing may be related to the fact I2C is muxed on Galileo, as some other IOs are.
Looking at the IO mapping document: Intel® Galileo IO Mapping I see that the I2C is muxed with ADC (very last line in the table) and by default it's switched to ADC.
So I think you need to do something similar to what SpiderKenny just described in his [very nice! :-)] blog post about using UART pins 0 and 1 to work with 1-wire devices: Using Galileo to read 1-Wire (such as iButton) devices.. Take a look at the source code he provides, there's a function for setting up the muxer to have the UART connected to pins.
Looks like you need to do a similar thing, but for just one GPIO #29, which needs to beset it to 0 instead of the default 1 and that will connect Quark's I2C SDA and SCL lines to pins A4 and A5.
In fact I can give you a c-source project that will work with the Adfruit Oled display.
I will try to attach it here.
Unzip it to a directory on your Galileo and then run 'make' and then run "./oledtest" and it should display the Adafruit logo and scroll it back and forth.
oledtest.zip 6.4 K
CedricAndreolli, great, glad it works for you now!
SpiderKenny, thumbs up, thanks for chiming in! Very nice blog post as I already mentioned, I was contemplating a very similar idea for a while and your post gives me an additonal inspiration to actually go and implement something based on the 1-wire DS18S20 sensor :-)
Although my code works mainly with the DS1990A, there is no reason why it shouldn't work with the DS18S20 too. Feel free to re-use any of my code in whatever manner you wish, the (C) notices at the top are just put there by Xcode which I use as my editor. I should really put a GPL notice or something there instead.
Good luck with the temperature sensor.