I have a similar unit. The outside unit won't come on. I can push in the contact and the fan and capacitor will come on. I can run a jumper from Y1 / common to O. It is like running a jumper from yellow to red in the picture above on the defrost board and the unit will run.

Can anyone let me know what I can look at?

If I push in the plunger on the contactor the fan runs. Moment I let it go it stops. So, this means I need a new contactor?

Not exactly. You need to check for 24v to the coil of the contactor. If you have 24v to the coil, then most likely you need a new contactor. If you don't have 24v to the coil of the contactor, you need to determine why you don't have the power to pull the contact closed. Odds are, your contactor is just fine, but there is not power to pull it closed.