What kind of beer is this? 18 months sound like a very large beer, which would be the only viable option for this. Even so, I'd be tempted to split this up into smaller batches and use several vessels where I could control the headspace and O2 absorption. Not only that, but you are stuck using a certain amount of oak. It might end up being too much. Using smaller vessels and adding oak chips would allow you to rack the beer off the oak when it's oaky enough.
I don't know. The thought of adding several small batches together, probably weeks apart, is a turn off to me. Even with purging with CO2 (Which would probably work just fine for the headspace, but the wood will leak some O2 though in 18 months time.)
I say find a 55 gallon primary and brew enough beer to fill it and ferment in that. Add that to the barrel one time and purge the headpsace. 18 months is a while, but not uncommon for certain very big beers. That's a lot of $$ to spend if something goes wrong. Is there a special reason you really need to go 59 gallons?