I know Ogrob completely answered your questions - but I do have some photographs...
I just bought a box of Oathmark human and I have a few questions about their use in Oathmark rules (Which I don’t own yet)
1. can I base the figures on round bases (25mm), I’ll then put them on Renedra movement tray.
2. how big are "units" in the rules? 10, 20 or 30 figures?
3, I was thinking of separate units for missile troops and melee weapons… in the rules, can men from the same units have spears and swords?
Cheers!
I am using Renedra 25mm round bases for compatibility with Frostgrave (my initial armies were all cold weather - though I have started an odyssey in warmer climes) and the Renedra sabot bases for Oathmark.

Bonuses are gained from the number of ranks in a unit (and making them more robust)
For human sized figures a rank is five figures (hence the sabot base size above) and a human sized unit can have a maximum of four ranks.
Larger figures on 50mm bases have three figures in a rank and can only have one rank.

You can only have four units of a particular species/weapon combination - so you can have four Human Spearmen, four human archers, four human soldiers etc.
This is not as limiting as you might think, because Oathmark armies are "National" rather than "Species".
The Army of the Wraith-Pixie AllianceRather than having a dwarf army, you have an army from a kingdom/dukedom/anarcho-syndicalist commune.
Your kingdom might have a human capital city, dwarfs in the northern hills, goblins in the western mountains and elves in the forests by the sea. You decide on the initial composition of your Kingdom, and that defines the availability of specific units.
A unit fights with one weapon type and has one defence type (and one activation number - that of their 'officer', so a unit of elves who have a Goblin General in their unit activate as a Goblin).
So a unit is Elf Spears or Elf Soldiers.
I think that most people would allow the "Front Rank Defines The Unit" subset of WYSIWYG.

Can you spot the odd two out?
There is a minimum point size for armies (some armies are extremely brittle at that point level).
Human Army (ranks rounded down)
Type | Ranks |
Soldier | 3 |
Spearman | 2 |
Archer | 4 |
Elf Ranger Army (complete, no really same points value)

Four ranks (and change) of Elf Rangers.