I know it's a straight forward build - everyone says that - i've built 2 O31 cases so my first question is: have you built any? It would be fun to talk to someone who has also has.
For example did you find 1/2" qtr. sawn oak 9" wide? or did you glue it? Did you grind 4/4 to 1/2" or did you re-saw? Did you re-saw from what? 6/4 giving enough for 2, 1/2" finished pieces? or 8/4 giving 2, 1/2 and 3/8 for drawer fronts? we're still milling lumber here, did any re-sawn pieces cup? (I re-sawed from 6/4
What joinery did you choose for the case? Finger or dovetail? (I chose dovetail
If finger joint how did you cut 1/4" fingers in 8.5"
How did you route the 6mm drawer slide slots in the sides and divider to keep them accurate so your drawer building went smooth? (I built jigs for my pin router.
I have built similar things. I own a custom furniture business, and have been at this for a while.
I am lucky to have access to 1/4 sawn white oak in most widths. As a rule, anything under 12" wide is fine as one panel. (there is a good reason hobby planers are 13" width). For a 1/2" panel it is best to resaw from 5/4 or 6/4 depending on how straight the workpiece is to begin with. 1/4 sawn material doesn't move too much, so I wouldn't worry about it. Woods that like to move the most are hard maple and similar. Oak is very stable, especially 1/4 sawn.
I'd grab the drawer front out of another piece of that 5/4 board. Don't resaw longer than you have to. If your case is going to be 20" tall, then resaw 23" or so, then resaw for the front.
I would also grain match the drawer fronts. You can grab the locking front panel and the fronts out of the same board.
Resaw tip, make sure you start with properly jointed stock. Achieve your final thickness through the planer or drum sander.
For something like that, I'd either use dovetails, mortise and tenon, or just tongue and groove. Either will be plenty strong for what you need.
Templates and a pin router is certainly a good way to go about it.