Background.
An orbital assault army in 30k is one in which all models must be capable of deep striking (and indeed must be deployed in that way). The army is therefore one of two things (or a mixture of them): a drop pod army where drop pods have been purchased in the place of dedicated transports like rhinos. Alternatively, they could be naturally capable of deep striking like assault marines. On top of this, terminators are immediately granted deep strike -- in 30k they do not automatically have this.
List Building & Evaluations.
I want to illustrate what is possible here. So I'm going to provide more than one list. This first one features ZERO drop pods.
HQ:
* Legion praetor in tartaros terminator armour, thunder hammer, iron halo (165 points)
Elites:
* Terminator squad in tartaros armour, 10 members, 3 power fists, 2 chain fists, 5 combi-plasmas (395 points)
Troops:
* Tactical squad, 20 members, sergeant with power fist and artificer armour, legion vexilla (285 points).
* Legion Assault Squad, 10 strong, all with melta bombs, sergeant with power fist and artificer armour (325 points).
Fast Attack:
* Storm Eagle with lascannons (250 points)
* 3 landspeedsers with multi-meltas, each with 1 hunter-killer missile (210 points)
* Primaris-Lightning Strike Fighter with twin linked frag and krak missile launcher, rad missiles, twin-linked auto cannon, phosphex bomb cluster (220 points)
I hope its clear that the storm eagle takes on board the 20 strong tactical squad, whilst the rest can fend for themselves. The list relies on applying pressure with the terminators, and with air support from the storm eagle plus primaris fighter. The land speeders aim to take tanks out whilst the assault squad back them up ably and takes objectives. I might be tempted to use a Caestus in place of the Primaris.
In this second list, below, we look at a PURE drop pod list, avoiding other fliers. And only taking the praetor in terminator armour as a singular threat who is able to join up with others as required.
HQ:
* Legion praetor, Terminator Armour, chain fist, iron halo, grenade harness (170 points)
* Master of signals, melta bombs (95 points)
* Master of signals, melta bombs (95 points)
Troops:
* Legion tactical squad, 10 members, 1 vexilla, sergeant with power fist, artificer armour (185 points)
* Dedicated transport drop pod (35 points)
* Legion tactical squad, 10 members, 1 vexilla, sergeant with power fist, artificer armour (185 points)
* Dedicated transport drop pod (35 points)
* Legion tactical support squad, 9 members, plasma guns, sergeant with power fist, artificer armour (320 points)
* Dedicated transport drop pod (35 points)
Elites:
* Legion veteran tactical squad, 9 members, 1 melta gun, 3 power axes, sergeant with power fist, artificer armour, all with melta bombs (280 points)
* Dedicated transport drop pod (35 points)
* Dreadnought Talon: 2 dreadnoughts with plasma cannons and chain fists (155 each)
* Dedicated transport drop pods (35 points each)
This list provides 6 drop pods, 2 of which are for regular dreadnoughts, and 4 for the tactical squads, support squad and veteran squad. The master of signals goes with the support and veteran squad to make the numbers up. On landing, they unleash their orbital bombardment, and / or give BS=5 to a nearby squad. The Praetor comes in via deep strike to pick off stragglers whilst the dreadnoughts give a fight where they're required. I personally wanted to try to take 7 drop pods, but to do so, I'd have to whittle down the support squad. Hence there's a need to be careful with the first wave and bring down the plasma guns, the veteran squad, plus probably one of the dreadnoughts to clear a landing zone upon getting on the board. Hopefully surviving the first turn of incoming fire, the rest will come in steadily to support them.