See if the OSB is rated for exterior use. If not it will probably fall apart if it is constantly wet. Possibly you could line the box with plastic film to keep the dampness off the OSB.

Hi madpo!

I've been raising tomatoes in 6gal black nursery pots, which tend to fry roots when left unshaded in the summer sun.
For this reason, since last season's end I've been trying to find a cost effective replacement. I looked at wood containers but it seems that even OSB boards will rot (according to wikipedia they aren't even meant for outdoor use) after a while and the green pressure treated stuff is treated with poison (arsenic) which tomatoes will tolerate just fine but you probably don't want to eat the resulting fruits on a daily basis for years...

I ended up settling on Spanish clay roofing bricks. Unglazed clay is gas-permeable, meaning water can evaporate through the brick, which is as close to air conditioning as my plants roots will get. A 2'x2'x4' container built with clay roofing bricks @ 0.24Â per brick ends up costing Â3.84 per planter, call it about 2 bucks a plant. Only downside is the labor involved putting the containers together with Silicon, and the fact that the end result is ridiculously heavy. Not sure if they're available in Wisconsin though!

As for soil I'd recommend checking out the container forum. Lots of enlightening posts on potting soil and aeration there.

Cedar is best, I think.And it is easy to contruct too.
I don't know exactly how deep the box should be to tomatoes but.
Two feet deep is not necessary. At the most I would do it 18 inches.
I would say 12 inches is probably enough. So, buy 1 by 6 boards and put two
of them together, by a ribs inside. same way with the bottom.
Use galvanized screws.
Predrill, to prevent cracking.
Can also use stainless steel or brass strips at the ends to look nicer.

osb doesnt hold up well in moisture / try some pine or douglas fir and use soybean oil to water proof if cost is a concern i use it and it lasts for yrs / cedar around here is way to $$$ for me

I frame all my plants with wood boxes since our soil is nasty compressed clay with rocks n them. I hate digging.

I use both cedar and redwood fence boards, whichever is on sale. They run me about a buck and change per 6' x 5" board. I stack them two high for tomatoes, cover the top with ~ 2 inches of compressed grass clippings then use doubled up ground cloth to hold the mounded stuff down. I cut a hole in the ground cloth to plant the young seedlings. Have drip irrigation.

This works very well in the dry Los Angeles valley heat.