If you got a riser for your 14" you could cut 10" - 12" for a resaw. You would want at least 1 1/2HP motor for "best" results.
That's a more economical way than a whole new machine. If you were going all new look for 12" resaw. Having said that I have a 19" 3 HP Grizzly and just looking at that blade raised 12" high is scary. I don't resaw more than about 10" myself.
Oak is not that "valuable" around here anyway to warrant resawing 12" boards. Besides what are you making that will require that width other than table tops?
Your best bet is to have a portable saw mill come by and let them saw the logs. I have done that 3 times so far and it's not that expensive, around here anyway.
If you have the 14" Delta cast iron saw it's worth a riser block at around $80.00. The Harbor Freight riser works from what I hear.
Your problem will be getting one flat surface to ride on the table or you will need to make a sled to secure the log from rotating. For shorter lengths (4 feet) I used a sled and rail underneath riding against the outside of the table for reference. Logs are heavy and you need in and outfeed support.
To make the one flat surface I used a power planer and a jointer. I find if you joint after each pass you have at least one straight and flat surface to ride against your taller fence.
Seems like it takes a lot of effort... and it does to set it up right. You don't want wavey cuts which will just waste the wood. And you need a planer to surface wider boards. And you need the widest jointer to surface them before running them into the planer.... and on ... and on.