Oathmark is pretty forgiving about moderate differences in unit footprints because only two opposing units at a time engage in combat and at the end of a round one or the other is moved back from contact. When units move into combat one or the other is moved to face the other edge to edge and they are placed centre to centre on the sides in contact, so having different unit widths doesn’t matter much.
Unit width differences would give figs on larger bases a slightly wider zone (and units with smaller bases a narrower one) for shooting, but that’s unlikely to have much impact on game play.
Possibly the greatest impact would be on movement- specifically units on larger bases having a harder time moving through friendly units ( the unit needs to be able to clear the friendly unit to move through). Units with smaller bases would find it easier to move through friends. A few minor rule tweaks would remove those effects, though.
The same applies to bases for large creatures- a bit of difference from the “official” base sizes is unlikely to break the game!
Overall, you probably wouldn’t have problems using 30mm or 20 mm bases as long as you and your opponent are willing to be reasonable about it!