P0138 - O2 sensor : Need help

[sbhuvana]

I have a pathfinder SE 2001, it showed a SES light last weekend and on getting the code checked, it came out as P0138 - Oxygen Sensor Bank 1 Sensor 2. I got the part number from the dealer as 22690-4W021 ( looks like the OEM Number) but the cost at nissanparts.org is around $125 and I can get this OEM on Amazon.com for $45.

Not sure why such a huge difference and which one I should get? You assistance is much appreciated.

Thanks,-Sri