What's the "single" derivation of omni?

Solution 1:

Note: I provided the following answer before the question was edited to suggest the same word. However, it confirms the suggestion.

What you're looking for is uni-:

[Merriam-Webster]
: one : single
// unicellular

In the example in the question, the complement of omnidirectional is unidirectional.

Solution 2:

Mono- could also be used.

A prefix that means “one, only, single,” as in monochromatic, having only one color.
It is often found in chemical names where it means “containing just one” of the specified atom or group, as in carbon monoxide, which is carbon attached to a single oxygen atom.

[Dictionary.com]