I will be installing oak 3 wide by 3/4thick hardwoods in my living room and dining room. I realize that I need an expansion gap of about 3/4 inch around the perimter, but my confusion is the following: Do I leave 3/4 gap between teh transition areas? I have a transiiton area going to tile, and one to linolium. Plus I will be framing in a fireplace and 2 floor vents, do I need any kind of gap around these areas? I will also be framing the fron of a sliding glass door, do I need any kind of gap in front of this, or do I run the wood flush? Thanks in advance for your input!

Yes you will need some expansion at all those areas. They are easily hidden with reducers, or t-molds. I think a lot of people will question the 3/4" though. NWFA sets that end-all standard based on a worst case scenario-FLOOD. This space is not meant for normal year-to-tear movement, it is there for worst case where the wood could possibly push the walls off the foundation. This i got out of their mouths last month at the convention. Lets face it, if a peice of wood moves 3/4', how is the cleat/staple holding it in?
A more reasonable way ould be to judge your climate, the species and most important, the width of the rooms.
Wood will expand across the grain far more than than with it. A wider room will need more space to move than a narrow room. A wider board will expand a little more than a narrow board. A humid climate will see more movement than a stable climate. All of this could be considered when there are problems hiding the expansion gaps. What is needed in Florida is different than whats needed in Chicago, or California.

Thanks Marco! I was kinda questioning the 3/4 gap, but think that 1/2 should suffice for 340sq feet in the Seattle area, not too much temp/humidity change here. Unfortunately, I have a newer house and they have OSB for the subfloor and will need to put down plywood..I was thinking of 3/4 plywood with about a 1/8 gap between the sheets, does this sound reasonable, or would you suggest something else? Thanks again for the info!

NWFA apparently now approves all USB as proper subfloor. This was not always the case though, it was only certain brands and mills they deemed usable. I've always wondered what changed their minds....so unless you can dig up another reason, save yourself the ply.
In the Seattle area, shops compete to give free advice. Call the Oak Floors of Greenbank, they are quite informative.

I install alot of unfinished flooring (3/4" T&G) over OSB all the time because the architects are specifying it in the building plans. I asked my builders why they are using OSB instead of plywood ( besides cost ) and they just said that was what the architect specified. So, the moral of this story is; plywood is better but OSB is just OK. It will work in an environment that is relatively stable. Staples do work better then cleats in OSB; at least initially. I have NEVER had a nailing failure over ANY substrate using staples. I have had them with cleats so PLEASE, lets not get into cleat vs. staples. Until someone can PROVE otherwise, it's staples for me over OSB. What I HATE about OSB is that the seams pucker more than plywood. That means more subfloor prep (sanding seams). If I am doing a 5/16" floor, I will install at least 3/8 ply because I don't think the OSB is going to cut it for that type of install. As Marco said, really no need for plywood in this case.

Thanks for the feedback regarding OSB, but I still don't get the warm and fuzzy's on installing over the original 3/4 OSB. I guess I would rather "waste" a couple hundred bucks on putting plywood over the OSB and have piece of mind that it will be a solid floor and last. I checked the condition of the OSB and looks like I would have much more prep work to do in order to get the seems level and get it in the acceptable condition, rather then putting ply down and calling it good to go. Last thing I want is a floor failure 5 years down the road and kick myself for not spending a few extra bucks to help eliminate it up front. Do you think 5/8 ply over 3/4 OSB would be more than sufficient?

What kind of OSB?!

Do you have Advantek down? Structurwood? Regular OSB?(like roof board)

If you want to stack it up, mo power to ya'. You probably don't need to, though. Do not ignore the fact that you are adding 3/4in solid oak into the mix. That , by itself, is structural integrity.

If you choose to cap the underlayment, make sure you install it in a structural nailing sequence. That means buying enough to put the end joints on a joist. You need to secure your decking to a joist everywhere you use a fastener. If you do not do that, you have wasted money and effort.

Good luck,
Chuck.

p.s. If you don't fix the probs at your existing side seams before capping them, you will have a more difficult prob to fix after you cap them.

It's a Princess/versus/pea sort of deal. Remove the pea.

If you want to install plywood, then 1/2" is the way to go. No need to use anything thicker. As Chuck says, you still need to address the high seams in the OSB by sanding flush. Then for MAXIMUM insurance, glue and screw the plywood down into every joist, as Chuck suggested. I wouldn't waste my time or money but you certainly are welcome to. BTW, I repaired and refinished a floor that I installed in 1990 over one of the first OSB subfloors I'd seen. That subfloor had been rained on, swelled up, dried out and I still installed the oak floors using staples. That floor was fine EXCEPT for the french doors leaking and the water damaged that ensued. The lesson here; OSB should not be a problem. Architects are specifying it all the time and the NWFA and NOFMA have decided it is an acceptable substrate.
http://www.nofma.org/installation1.htm
http://www.nwfa.org/member/pdf/Sect2.pdf

OK, you guys are the professionals and I don't always believe what I read in the books, but if you guys are installing over OSB (staples) and not having any issues then I feel better about doing it, how can I tell what kind of OSB that I have installed, and should I not install over any certain type of OSB. Thanks for all of your advice, I'm just trying to do it right the first time, I never have installed any hardwoods before, as you can tell:)

There should be a grading/specification stamping on the panels. While sometimes they get installed upside down, you still should be able to find the stamp on at least some of the panels. It will give the manufacterer, size (in 32") should be 23/32", APA approval for subfloor, date of manufacter,etc. Record that info then check with the APA if that is an approved OSB subfloor. If approved, then your good to go.