Where is there a list of best BAA or OBA for pitchers, single season?

You can find it on Baseball Reference, but you'd need a Play Index Subscription.

Here are the top 25 for both, only counting pitchers who qualify for the ERA title. Note that there are a lot of 2015 guys, but they will not keep this up for the entire season. The stats date back to 1938. The left side is the top 25 for BAA, and the right side is top 25 for OBA. Not counting 2015, the lowest ever BAA was posted by Luis Tiant in 1968, .168. Nolan Ryan has seven seasons with a BAA .190 or lower. Sam McDowell has three, and Koufax and Pedro both have two.

The lowest opponent OBP was posted by Pedro Martinez in 2000 - .213. Kershaw's .231 last year was the fifth best of all-time (since 1938, when they started keeping track). Currently, Dallas Keuchel's .228 this season is fourth best, but that will increase after he pitches more innings.

I'd check fangraphs.com they have a lot of advanced statistics on their website. It's my favorite website for checking stats. BRef is pretty overrated in my opinion

It's probably here somewhere http://www.baseball-reference.com/ but you'll have to do the looking.

Not there.