Jep & Joe,
Jep, from what I can gather it sounds like your framing should be strong enough for ceramic tiles, even though you didn't mention the span of those timbers. 1x3 planks should be good for 24" o.c.
The 1x3's are your subfloor, the OSB is the underlayment. I don't like 1/2" OSB cuz it's usually a cheapo grade, very absorbent material used mainly as sheathing or utility such as shelving. I don't trust bonding to it.
So, If everything is installed correctly and in good condition you can keep the OSB and install 1/4" backer as directed into fresh thinset, taped etc. Otherwise add 3/8" or thicker B/C underlayment ply then either 1/4" backer or Ditra. http://www.schluter.com/6_1_ditra.aspx
Ditra is what you want for floors, not Kerdi as Joe thought. Save the Kerdi for your next shower installation.
I would prefer to just screw the hardibacker into the OSB and not use thinset.
Where did you get that idea? That would be a very bad thing to do. I guarantee at minimum grout cracks within a few seasons.