It just finished the complete 26-ep run last night. It was supposed to end the Sunday before, but a screwup resulted in episode 20 being aired instead.
Anyway, The Big O was originally written as a 26-ep series, but they only got funding for the first half when it originally aired in Japan. Unfortunately, ratings were low and the second half wasn't funded. The first half eventually made its way here, where it became a bigger hit on Cartoon Network in one of the Toonami slots. Enough buzz and support built up here that Cartoon Network threw in the cash to finish off the series (hence, its being listed in the season 2 credits as co-producer). The second half aired at the beginning of this year in Japan, and premiered here in August, finally ending last night.
I can't wait for the DVDs to start coming out here starting in January. My poor VHS tapes of the CN airing are getting worn out due to repeated viewings, after all.