Mostly look for bottom contours up and down the 100 fthm curve. Examples include The Point, Tuna Hole, etc.
Occassionally we will fish inshore a little bit to around 50 fthms or out a little more, but for tunas, generally the 100 fthm curve is the ticket.
Make sure your Chart Plotter is set-up to read Loran TD's. This is my biggest recommendation fishing out of OI. The fleet down there uses them almost exclusively to mark location. Generally, if a single TD is given (ie "I'm on the 560 line"), then they are likely where the 560 line crosses the 100 fthm curve.
Good luck down there. We fish out of there almost exclusively and would be glad to answer any questions or help out with whatever you need.
Ed
Depending on time of year, it can change dramatically.
For Bluefins, I would look for bait/marks or good signs in 20-30f and deeper. My gut tells me these fish are "driven over" a lot by folks running out for Yellowfins/Gulfstream water. I say this because most of the Bluefins we get just a bit to the north of OI come in 20-30f EXCEPT in May (our early season) when you will get them closer to the Canyon and deeper water...
For Yellowfins, I think time of year plays a big part. In April and early May, the break will most likley be very defined and I agree, I'd look for where it crosses the 100f mark...but always explore areas from 50f on out that look good. As May wears on and you get into June (break should blend more), I would say definitely look from 30-40f on out and many times, you'll find the charters fishing inshore of the break (in the colder, green water) and using planers to get down deeper...and they will be very successful with this method.