Just bought Cyberpower OR700 1U UPS (20lbs). Will I need to buy rack mount rails to also support the back or will the cage nuts at the front be enough to support the UPS?

The product manual for the OR700 indicates that it comes with a set of rack mount brackets. A bit of searching with Google Images shows that these brackets are only supported by the front rails.

Generally speaking, a product's official rack-mount hardware is sufficient to support the product on its own, without anything pressing on it from above, and assuming all four screws (probably not included) are secured into cage nuts of the matching size and thread.

From my experience, 8 kg (18 lbs) for a device which is only 23 cm deep (9.2 in) is no cause from alarm, when installed with all 4 screws. Heavier appliances exist which also don't require a 4 post rack.

I suspect you’ll need rails.

My server is in the 25# range and it has rails.

Some of my networking gear is in the 4-10# range and they don’t have rails.