@Vaclav-VESELY Probably my phone charger was giving out less than 5V and that affected my O2+ or the DS18B20. Now I'm using a Meanwell RS-15-5 adjustable power supply (http://www.meanwell.com/webapp/product/search.aspx?prod=RS-15)

OK, I got it working. I had to change the power supply from a phone charger to a 'real' power supply.

My first thought was exactly that. So I redid the wires before posting. Same result.

I did a check just now. The data pin is not connected to the omega, just the ground and the power and I still got the weird devices:

root@omega:/sys/devices/w1_bus_master1# ls -1d ??-*
00-400000000000
00-c00000000000

And I just did a retest without anything connected to the omega and I still got the weird devices:

root@omega:/sys/devices/w1_bus_master1# ls -1d ??-*
00-400000000000
00-800000000000

It is still not working for me. I just upgraded to the b151 firmware. I got the directory /sys/devices/w1_bus_master1, but I got slave entries like:

root@omega:/sys/devices/w1_bus_master1# ls -1d ??-*
00-100000000000
00-900000000000
00-e00000000000

And it keeps changing. But I never see the correct serial number of the device.