Two points to note before I suggest something:
- I don't know the Ooznest hardware kit at all, so not sure how the stepper motor to driver connections are made (ready-made harness and connectors? Bunch of wires that you install yourself?)
- I don't know what control software the Ooznest uses, so not sure exactly what configuration facilities there are.

Having said all that, this is not at all an uncommon problem with a newly-built CNC machine, and there are two standard fixes, either one of which (but don't use both!) would work. However, bear in mind my opeing remarks - these suggestions are generic and I do not know how they might apply to your machine.

1. Swap two of the stepper motor wires over. Assuming the stepper has 4 wires coming out of it (some have 8, which makes it slightly more complicated), they are in two pairs, usually going to terminals on the stepper driver labelled A+/A- and B+/B-. Either swap the two A wires with each other, or the two B wires with each other. Do not swap both pairs. This reverses the direction of the stepper motor and it should work.

2. In the control software, find the configuration section. With the usual control software (Mach3, UCCNC, or similar) there is a configuration page which gives you access to a lot of different settings. For each of the three axes, there are a few parameters, one of which will be labelled "reverse" or something similar. If it is set, then unset it, or vice versa. Again, this should reverse the motor.

My guess is that no 1 is your best bet, assuming that the hardware did not come with a ready-assembled wiring harness that cannot be easily modified.

Good luck - but be reassured that the fix is likely to be very simple and this is not at all an uncommon problem.