Yes, as you observed, esc key became the system default shortcut for spelling correction. We therefore moved the key behavior you are missing to the "enter" key. Note that that's not the same as the "return" key. It is, however, the same as fn-return.
We think it's best to leave it that way, but if you disagree, drop intro a terminal window and execute this command:
Code:
defaults write com.omnigroup.OmniPlan OOShouldToggleEditingWithEscapeKey -bool true
Then relaunch OmniPlan.