Hi,
It's sure possible but it wouldn't be easy and you'd have to study flat out on the days you are not at work. There will be some weeks when deadlines clash so you'll have to be really organised and motivated. I did 60 credits a year and worked full time, so I'm sure part time work and 120 is possible, and plenty of people do it, but it's hard. You can't really have a slack week! The OU may contact you to discuss it if you do register for them all.
Hope that helps! What subjects are you thinking?