We might consider both Halladay and Oswalt to be among the royalty in pitching. Some of the data I present below suggests that.
The first thing I did was to find all the pitchers through age 32 with 1500+ IP since 1900 and rank them by RSAA/IP (there were 445 pitchers). RSAA means "runs saved above average." It is from the Lee Sinins Complete Baseball Encyclopedia. It is also park adjusted. This is through 2009. The table below shows the top 10:
The next table simply shows total RSAA.
Then I found the leaders in pitching Wins Above Average (WAR) from Baseball Reference. Here are the leaders through age 33 including 2010:
I also constructed a crude fielding independent ERA. I ran a regression with these pitchers (through 2009) where their relative ERA depended on their relative HRs, SO, and BBs. Then I used that regression equation to predict their relative ERA (if I get a chance I will add the results-the r-squared was about .58). Here are the leaders:
So Walter Johnson had an ERA that was 69% better than the league average and he was 69% better at preventing HRs. He was predicted to have an ERA that was 48% better than average (but none of these numbers are park adjusted). Both Roys do well again.
Notice how they are alot better at preventing HRs and BBs than average but just slightly above average at striking out batters. That is similar to Maddux. In fact, I have created a HR/BB index for pitchers that Maddux did very well on. See Who Was More "Magical" Than Greg Maddux? (Or Pitcher's HR/BB/SO Rating).
I also created a WAR ranking using this crude fielding independent ERA. I divided IP by 9 to get games. Then adjusted every pitcher to a league average of 4 runs per game. That gave Walter Johnson an ERA of 2.74. Those numbers were used to calculate a predicted winning pct using Bill James' "Pythagorean formula." To compare that to a replacement level pitcher, I assumed that would be a .400 pct. So if a pitcher had 200 games and a predicted pct of .600, he would get 120 wins. The replacement would get 80. So the pitcher in question would have a WAR of 40.
The two Roys did not rank as highly as they did in the above tables, but they were still pretty good. Halladay was 50th and Oswalt was 68th. That still puts both in the top 15%. Walter Johnson had a predicted pct. of about .685 and a WAR of about 134 to lead all pitchers.