Okay, the new best method for plotting circles is to use the attached AppleScript.
This AppleScript and corresponding python script calculate a number of points in a circle given that circle's center and radius, and paste those points into OmniGraphSketcher using system events. The default number of points is 36, and can be changed by editing the variable "segments" in line 7 of circle.py. To connect the points plotted by this script, use the "Connect Points" button at the bottom of the Data inspector.
To add as a toolbar button in OmniGraphSketcher, place the entire Circle folder in the directory ~/Library/Scripts/Applications/OmniGraphSketcher/ (where ~ is your user directory), then add the button using View > Customize Toolbar...
The the .scpt and .py files must be in the same directory for the script to work. Feedback is welcome; please email OmniGraphSketcher@omnigroup.com
with your questions or concerns.
edit: outdated script removed