Oblivion is NOT an action game, and that is not an opinion or a negotiable statement. It is a fact. Character stats and character planning have NEVER been more important in the series, and yes, it is flawed, but it, undeniably, makes the game an RPG.
I have to disagree. I consider the series as a whole to be "sandbox action-RPG". You tend to take it upon yourself to be Oblivion's champion and defend it against all perceived criticism, but you're really stretching it when you argue against its action elements.
There are 21 skills in Oblivion. Of the 14 non-magic skills, 5 are not explicitly combat-based; athletics, security, sneak, mercantile, and speechcraft. Security and Speechcraft are nearly useless due to being dominated by their respective minigames, the latter made more so by a lack of any important or difficult dialogue situations where you might need it. Half of sneak's function is combat-related, and 3 of its 5 perks are as well. Mercantile is useless thanks to a nonexistent economy, and the fact that it's raised only by selling things (NOT by haggling or anything else, though "anything else" pretty much doesn't exist), making it function pretty much the same for all characters. Athletics doesn't have a lot of point with fast travel, and if you're not inside a city, almost all locations you can possibly be walking to are monster-filled dungeons.
Of the 85 spell effects in the game (not counting restoration, or alchemy, which can functionally replace almost all skills), only 10 have any use outside of combat; chameleon, invisibility, light, night-eye, detect life, telekinesis, feather, open, water breathing, and water walking. Even then it occupies much of their use; if you're not using invisibility to be unseen doing something illegal, escaping monsters is about the only other use. I've never used light/detection spells outside of dark dungeons to better see what I'm fighting, or feather for anything but carrying the loot I got from all my dungeon-diving. Restoration's extra 25 effects can largely be used anywhere, but again, the majority of their function is combat. You only catch diseases in combat. Stats (and thus fortification) have almost no use outside of combat. The vast majority of health loss, and subsequently healing, comes from combat. So yes, I can negotiate your opinion on whether it is an action game. Oblivion has a very,
very heavy emphasis on combat.
Planning, in your words, is important because raising your level with non-combat skills will cause the enemies that scale to become too difficult for your non-combat character. The argument that this somehow makes the game more of an RPG is completely empty. It's poor balancing and nothing more. The average FPS, obviously, expects you to pick up a gun and start shooting enemies. The fact that you can avoid guns and ammo and as a result probably die unpleasantly does not make the game an RPG. I consider player choices to be vital to whether something is an RPG. That awkward scaling reduces choices by punishing you for straying from specific options. This does not help the game as an RPG. That you would call them "unforgiving RPG mechanics" is baffling. Any genre can limit your options with crippled mechanics.
Now, Morrowind is much the same here. I can't speak for Daggerfall or Arena or the non-numbered games. There is stealing (most of which is for loot that's either useful in combat, or can be sold for money that can be spent almost solely on items useful in combat), exploration (for either interesting things to look at, or dungeons), and combat. Not much else. I would say that just about the only reasons I'd consider Oblivion to have a higher combat emphasis is less dialogue (albeit equally unimportant), fewer cities (thus less stealing), and the minigames that make it pointless to train in the few non-combat skills available. Much of that, even, are balancing matters more than fundamental changes.
By all means defend Oblivion from unfair attacks, but I don't think an action emphasis is one of them. Claimed to have far more of one than Morrowind, sure, but not in general.