As an Ono, I chose Carapace and Adrenaline.
Defense- Carapace because any Kharaa who gets shot with a lot of bullets needs this.
Movement- Adrenaline because I can bite faster, charge longer and recover from battle cries more quickly.
Senses- I do not upgrade my senses until I get low on health. I will get healed to full in 5 seconds.
Gore- I bite fast moving marines. Remember to aim the center of the screen at your target (usually 45 degrees down)
Paralyze- I use this when sneaking up behind marines. (Yes, Onos can do that) In my experience they can always shoot back through. :\
Charge- Here's a nifty one that you all should know, so I will bold it.
You can use Gore while charging. Also, charging hurts things at your cross-hair, so aim down. When attacking structures, crouch and hold forward while biting. I am pretty sure using charge repeatedly does nothing.
Primal Scream- It drains all your energy, so remember to wait for all of it to return before making any attack. With adrenaline, it would only be a few seconds. It will help both Gore and Charge, so remeber to use it before making any serious attack.
Defensive tactics:
Nothing soils more pants than a cloaked Ono. Though less dramatic, it is best to ambush from behind with an intial Paralyze. Don't do the scream first, unless you just want to frighten the poor guy.
Offensive tactics:
Hit and run is the best. Scream, rest, run in charging and goring the turrets or undefended turret factories. Ignore players for the most part. They are expendable, the structures are not. Run before your armour drops too low. Remeber that Onos die faster that Lurks if they run our of armour. When you need health, run out and evolve your senses for a free heal, get a gorge to patch you up, or stand by Defense Chambers or Hives.
Real Onos only press W. Avoid strafing and backpedling. They slow you down and do not deal damag with the charge.
Onos can fit through suprisingly tight places if they crouch. Never jump on Kharaa structues, though it is safe to run across the top of marine structues. Try this. Strafe into a wall, now crouch and try strafing closer. You will be suprised. Use this to get around obstructions. Hug the wall.
Ignore tripmines and grenades. A charging Ono who has just heard a Primal Scream can avoid most damage from the trip mines. As for grenades, there are other things that will hurt you more. Like HMGs.
Whenever you run out of energy, it is best to rest. Either fall back and get healed or stand still and cloak. Simply sitting and biting once a second is quite ineffective.
Gorges are your friends. Have them heal you and build Defense Chambers whenever possible.
Lerks are also your friends. Have them fly in and land on Turret Factories and start spitting out their magical gas. You will be able to destroy it nearly unscathed.
Have Fades or Lerks attack marines from a distance while you have your way with the marines. They will keep to dreded HMGs off your back and even help you in destroying structures. Remember that turrets attack the closest enemy.