One way to get to the sanctuary is to drive straight down Nizamuddin Bridge, turn right towards NOIDA (which means turning left onto the cloverleaf that goes up on to the flyover to NOIDA), drive straight down past the UP border (and the Gautam Buddha statue) and on until you reach a sign announcing the entrance to the DND Flyway. (If you take the DND Flyway from Delhi instead, exit on the left towards NOIDA Sector 15 and Mayur Vihar, take the first U-turn, and you'll be at this sign.) Drive on under the flyover and take the first U-turn. Park in front of the park to the left, and walk up to the river by following the dirt track that runs alongside the approach road to the flyway.
The other route is to go straight down NH2 (Mathura Road) and turn left under the flyover just after Apollo Hospital. Keep going straight down, turn right at the end of the road, and you'll be at the Okhla Barrage.
See this map for an overview of the area.
Here's a brief checklist of the birds I've conclusively identified at the OBS:
The Delhibirds web site has a Yamuna checklist compiled by Clive Harris, but it's not actively maintained.
Abhijit Menon-Sen <ams@toroid.org>