Originally an actor in a Chicago theatre company, Ostrander moved into writing comics in 1983. His first published works were stories about the character "Sargon, Mistress of War", who appeared the First Comics series Warp!, based on a series of plays by that same Chicago theatre company. He is co-creator of the character Grimjack with Timothy Truman, who originally appeared in a backup story in the First Comics title, Starslayer, before going on to appear in his own book, again published by First Comics in the mid 1980s. First Comics ceased publication in 1991, by which time Ostrander was already doing work for other comics companies (his first scripts for DC Comics were published in 1986). Prior to being a writer, he actually appeared as a supporting character in the Supergirl comic. His friend Paul Kupperberg was the writer and incorporated him into the Supergirl titles relaunch in 1982.
Prior to his career in comic books, Ostrander studied theology with the intent of becoming a Catholic priest, but now describes himself as an agnostic. His in-depth explorations of morality were later used in his work writing The Spectre, a DC Comics series about the manifestation of the wrath of God. His focus on the character's human aspect, a dead police detective from the 1930s named Jim Corrigan, and his exploration of moral and theological themes brought new life to a character often thought of as impossible to write. He has also worked on Firestorm, Justice League, Martian Manhunter, Manhunter, Suicide Squad, and Wasteland for DC.
From the mid-1980s until her death from breast cancer in 1997, Ostrander frequently co-wrote with his wife Kim Yale. It was while working with her that he made what is probably his most lasting contribution to the DC Universe: the recasting of Barbara Gordon, the former Batgirl, into the information and computer specialist Oracle. He is also responsible for the creation of characters like Amanda Waller, Shango, the Soyuz and Pozhar.
At Marvel Comics, Ostrander has also worked on X-Men, Bishop, Quicksilver, Heroes for Hire and the Punisher, as well as the acclaimed Western mini-series The Last Ride of the Western Heroes.
Ostrander has also written for other comics companies: Hotspur for Eclipse Comics; Lady Death for Chaos! Comics; Magnus, Robot Fighter, Rai and the Future Force and Eternal Warrior for Valiant Comics.
Ostrander was one of the main writers on Republic for Dark Horse Comics, and many of his story arcs, such as "Twilight", "Darkness", and "The Clone Wars" stories are available in trade paperbacks. He is also the writer of an ongoing Star Wars comic series, Legacy.
Ostrander has also written a Doctor Who audio drama for Big Finish Productions. Titled Deadman's Hand, the story was originally announced for release in March 2004; it has since been resheduled twice, but no longer appears on Big Finish's pages as a Future Releases and now seems unlikely to ever be released. As announced, the story was to feature the Seventh Doctor, Ace and Hex.
His work has won him a good deal of recognition in the comics field, including nominations for the Comics Buyer's Guide Award for Favorite Writer in 1997, 1998, 1999, and 2000.
In December 2006, DC Comics published a new Batman story-arc titled 'Grotesk' written by Ostrander that ran through Batman issues 659-662 with Tom Mandrake drawing.
Ostrander has written issues of Secret Six with writer Gail Simone.
Ostrander suffers from glaucoma, and is facing significant bills in treatment. To help cover his costs, a benefit auction has been organized for the 2009 Chicago Comic Con, with donations being managed by the website comix4sight.com.