'I was expecting to do well,' said Harrington. 'I knew that a lot of people would have to shoot well to keep up with me.'
Otto closed with a 5-under 65 to take second place. Thomas Bjorn, Darren Clarke, Chris Gane and last year's champion Fredrik Jacobson shared third place at 7-under-par 273.
Harrington started the day one shot behind overnight leader Christopher Hanell. Harrington took control in the middle of the round, but needed a late rally to secure the title.
The Irishman birdied the second, while Hanell bogeyed the hole to give Harrington the lead at minus 9. Harrington maintained his one-stroke lead with four straight pars.
Harrington, the 10th ranked player in the world, birdied the seventh to grab a two-shot cushion. Hanell climbed back within one shot with a birdie at No. 8.
'I went out the first nine holes real focused and hit the ball real well,' said Harrington, who earned his eighth European Tour victory. 'I hit all the fairways and all the greens. It was as easy a 3-under par as I have ever shot.'
The 2003 Deutsche Bank - SAP Open winner fell back into a tie for the lead with Hanell as he bogeyed the 10th when he was unable to get up-and-down for par.
Harrington regained the lead by himself when Hanell dropped a shot at the 11th. The Irishman drained a five-footer for birdie at No. 12 to extend his lead to two shots.
He later missed a short par-saving putt at the 14th to drop to minus 9. Harrington was joined there by Otto, who birdied the 16th to get to 9 under.
Otto rolled home a birdie at the 17th from within 10 feet to climb to 10-under and take the lead. Harrington, standing on the 16th tee, saw that he had been caught.
'I didn't particularly like the tee shot on 10,' Harrington said. 'Then I was just trying to stay ahead of Christopher (Hanell) for the next couple of holes. It was when I got to the 16th tee that I realized Hennie had overtaken me. My mind set changed totally and I just started to play aggressively from there in. '
Otto's birdie try at the last slid by the right edge and he closed at minus 10. Harrington then flew his second shot over the green on 16, but nearly holed to chip as he managed to save par.
Harrington dropped his second within 15 feet of the cup at the 17th. He rolled home that birdie try to tie Otto for the lead. The Irishman found the fairway off the tee at the last.
His second came to rest about 20 feet right of the hole. He calmly rolled in that putt for birdie and the win.
'I got back into my focus the last few holes, which is nice,' said Harrington. 'It's nice to be able to turn it around like that.'
Otto began his day at 5-under par, three shots off the lead. He birdied the first and sixth to move up the leaderboard. He got to 8 under with a birdie on No. 13.
A bogey at the next hole seemed likely to derail Otto, but he then ran off three consecutive birdies to forge into the lead. However, he could not birdie the last and finished second.
Hanell could only manage a 2-over 72 in the final round. He finished in a tie for seventh at 6-under-par 274. He was joined there by James Kingston (67) and Prayad Marksaeng (71).
Jose Maria Olazabal and Michael Campbell both posted rounds of 4-under 66 to climb to 5-under-par 275. They were joined in a tie for 10th by David Carter, Rob Rashell and Gary Rusnak.
Richard McEvoy, who held the lead after the first two rounds, finish at 4-under-par 276. Also finishing at minus 4 were Steven Jeppesen, Henrik Nystrom, Stephen Dodd, Nick Faldo, Marc Pendaries and Nobuhito Sato.