[Baalz]

Fighting Oceana: The primary threat here is going to be the knights of the deep. Without some maneuvering on your part they’ll fairly easily handle everything you can field in the early game. They’ve also got some fairly tough heavy infantry that will be supporting them. The good news is that they’ll usually be investing in a heavy bless, so they’ll be lacking in other areas. They’re unlikely to be able to have the same income you do, and are also unlikely to have an awake pretender. Obviously I’m not going to be able to cover every possible thing you could encounter here, but your advantages in this matchup:

You’ll have many times the troop numbers that Oceana has. You can recruit more than 8 deep ones for the same cost as a single knight of the deep.

You have better early combat mages. Oceana basically is limited to water magic.

You’ll probably have a significant encumbrance advantage. Water blessings are common, and that heavy infantry has heavy armor. Cold-3 is very common for UW nations, and indeed you’re taking it yourself.

You’ll probably have a decent initial research edge as Oceana recruits priests to lead their sacred troops, confident they can lean on them and don’t need as much early research.

So, with the early research targets I suggested you’ve got the following in your arsenal early (I’ll assume level 4 alteration and conjuration):
Krakens & Sea serpents
Body ethereal, quickness, luck
Boil, numbness, destruction
Summon earthpower/water power
School of sharks.
Wolven winter
Encase in ice

Again, what you do is going to vary as you and your opponent try to outmaneuver each other, but it shouldn’t be too hard to see that if you play your cards right you can do a hell of a lot with not many mages and a mess of deep ones. Make sure via wolven winter that you’re fighting in cold-3. Send out schools of sharks to soak up lance attacks. Use boil and numbness to soften up the knights, and then send the buffed monsters as the muscle to take a bite. Sure, knights will run right over krakens if you send them right in, but if you wait until after they’ve plowed through sharks and your PD in very cold water, are getting tagged by numbness, and are just are hitting your fresh deep ones to come on the flank with a couple quickened, ethereal, lucky krakens those sea monsters look a lot scarier. Then see how well that fancy Oceana heavy infantry fares when they’re outnumbered and you start tossing destruction out there. You’ll have iron bane soon enough, but I chose alt-4 as what you’re at now.

Speaking of alt-6, you’ve got a pretender who can drop darkness immediately if you’re pressed, without needing to do any conjuration. Under darkness Oceana has almost no chance. Once you b-line to alt-6 I have a hard time coming up with much that Oceana can hope to do under darkness and iron bane…but, obviously I’m wrong because as has so clearly been shown by Zeldor you have no chance to win against Oceana.