A player wants his bard be a revolutionary of sorts, riling the populace to rise against their oppressors, but I'm not sure if the skill the bard should use to do that is Perform (oratory) or Diplomacy as both seem valid candidates. Which one is it?
Perform (oratory) typically only raises money
When a creature makes a Perform (oratory) skill check, the creature's attempting to scare up some pocket change by entertaining the audience with its speech. The effect that entertainment may have on the listeners is beyond the orator's control: the DM decides if there even is an effect at all, likely basing his judgment on the player's description of his PC's oration, the general attitude of the orator's audience, and the Perform (oratory) skill check's result. That is, the oration's content isn't really important mechanically—the amount that the audience donates to the orator's cause depends solely upon the Perform (oratory) check's result, making it possible for an 8-hour speech about the horrors of the current government to elicit as much in donations as an 8-hour speech about the myriad of fascinating ways paint can dry.
(The Player's Handbook on the skill Perform says, "Trying to earn money by playing [i.e. performing] in public requires anywhere from an evening’s work to a full day’s performance" (79), hence the 8 hours figure above. This DM reduces the amount earned with a Perform check commensurately with the time spent performing if performing less than 8 hours. So, for example, a 1-hour check yields 1/8 the normal amount, a 10-min. check yields 1/48 the normal amount, and so on. Also, Perform checks that are made by folks other than bards and that are not made to earn money are, apparently, a thing, but they've no mechanical effect in the core rules.)1
Diplomacy changes attitudes toward the diplomat
On the other hand, successfully using the skill Diplomacy allows the diplomat to "change the attitudes of others," with one of the skill's introductory examples involving convincing "the ogre mages that have captured you that they should ransom you back to your friends instead of twisting your limbs off one by one" (PH 71). So, by default, the skill Diplomacy can work on groups of creatures, although it's unspecified exactly on how many it can work against at once. In addition, a successful Diplomacy skill check made to change one or more creatures' attitudes only potentially changes those creatures' attitudes toward the diplomat—not toward, for example, an individual, a group, or a cause. The skill Diplomacy can make listeners like you, but it won't make listeners like stilton or Equifax.
However, listeners that like the diplomat enough may be willing to follow him in his crusade. That is, folks that possess toward the diplomat an attitude of at least friendly will provide him limited aid, folks that are at least helpful will take risks to help the diplomat, and folks that are fanatic are, for a time, ready to die for him! Again, though, it doesn't matter the diplomat's cause. The diplomat has merely persuaded his audience that he's a freakin' boss, and whether the diplomat's cause is saving Delta House from Dean Wormer, making folks believe he's doing the right thing for Teldar Paper, or rebuilding after a devastating meteor strike, folks will—if the Diplomacy skill check is high enough—do stuff for him and not because they have any particular attachment to what he's talking about.
Using Perform instead of Diplomacy
The Epic Level Handbook on the skill Perform says that the Perform skill can be used to "sway an audience's attitude with your performance" (42) then presents again the same kind of attitudes chart as is included with the skill Diplomacy except that the DCs to sway an audience's attitude using the skill Perform are 20 higher than those needed to sway the identical attitudes using the skill Diplomacy! (The SRD instead repeats in the epic Perform skill description the chart from the epic Diplomacy skill description. The SRD's handling of epic skills keeps 3e legacy text (q.v. the epic skill Handle Animal) and—in this reader's opinion—appears to've been hastily compiled. So, although it pains this reader to say it, his instinct is to trust that the Epic Level Handbook DCs are, in fact, accurate rather than ascribing stealth errata to the SRD.)
No further information is given about what, exactly, swaying an audience's attitude means, so this reader assumes—given the presence of the Diplomacy skill attitude chart—that doing so otherwise works like the Diplomacy skill: because of the performance the audience likes (or dislikes) the performer more rather than because of the performance the audience, for example, now likes (or dislikes) cats or the harpsichord. Also the text is silent on how long it takes a performer to sway an audience, forcing the DM—if he opts to use these rules at all—to make that decision.
What the player may actually want
What I suspect the player really wants—because it's awesome—is for his PC to be able to change an audience's attitude toward things other than his PC; he wants the PC's audience to, for example, embrace or loathe a particular art, food, organization, or pet because his PC said to. That is, the player doesn't care if the audience likes or dislikes his PC but if the PC's audience loves or hates the subject of the PC's rant.
That's possible, but that's not a normal use of the Diplomacy skill nor of any Perform skill. The DM must either devise house rules to enable the diplomat or performer to do that or allow into the campaign feats like the ancestor feats Renowned Courtesan (Dragon #315 65) or the superior Silver Tongue (Oriental Adventures 65–6 yet the feat's benefit's changed by the Dragon #318 article "Oriental Adventures Update: Eastern Flavor" (32–48))—both only available at level 1—or feats like Seelie or Unseelie Court Noble Kelir (from the Fey Feature Web column “Life in a Noble House”)—both having difficult-to-meet prerequisites or mandating a convoluted PC background.
Keep in mind, though, that, as written, such feats in the right campaign (and, arguably, in any campaign) are extremely powerful. Being able to mundanely and forever twist the hearts and minds of other creatures so that they love or hate what they previously hated or loved has profound consequences on the narrative landscape. Seriously, if PCs have feats like these, other folks in the campaign must have feats like these so that there's at least the illusion of stability; without that balance, the PCs will quickly raise an empire… or hurl the campaign setting into anarchy.
1 For those wondering why this DM even has a house rule for this, the more-adventurer-friendly skills Sleight of Hand and Tumble can also be used to earn money like the skill Perform, and those're what this DM's seen PCs use more often. That is, sometimes PCs are at loose ends—stuck waiting in a town for a magic item to be created, for example—and, to pass the hours or days, the PCs use their skills to earn pocket change. Usually, it's enough to overnight at a nice inn so the player doesn't have to erase gp from his character sheet.
The higher one
When a character has two skills that are equally valid for a given check, in games where the GM is telling the player what skill to roll, you ought to use the higher one. For a bard this is probably Perform(Oratory), but maybe not.
This is in part because you ought to be working with, rather than against, your players at the meta level of the game, but also because choosing the other skill does nothing but slow the game down.
If both skills are roughly equally applicable to what you think the player has described and you choose the lower skill, the player will just alter their action at that point, or in the future if you force them not to alter it immediately/retroactively, to instead select the higher skill. Essentially you are just making them guess what you've secretly made up as the code to get to use the skill they picked, and that's not usually conducive to having a good RPG.
This is DM call. Here's how I work it as a DM. The most basic answer to your question is actually a question:
What is the character doing? Are they making a speech a la "I have a Dream" in front of a large gathered crowd? That would be Oratory. If they specifically call out someone in the crowd--diplomacy, a la "To Kill a Mockingbird" when Atticus talks down the mob by getting specific and addressing them as people rather than as a mob/group.
Diplomacy is about making contact with decision-makers and engaging with them. Oratory is about the general mood.
So it might be a combo. Your speaker starts with Oratory. Maybe it works on the general crowd, a bit, but some of the "taste-makers" or leaders in the room remain skeptical. If he speaks to them specifically or asks them what their issue is, addressing it, that will be diplomacy. A great roll on that will help boost the oratory, as they go back to addressing the crowd...and so on.
As always, this is DM call, but I use those rules of thumb as far as determining which they are using.
Perform Oratory seems to the be best fit, if you don't want to do all that. Oratory is about moving an audience, diplomacy isn't as much. Keep in mind that with Oratory he can change the mood of the crowd but he won't have much control of what they do. So you can rile people up, but getting them to take a specific action is harder. It can lead to the people burning down their own town rather than going to the Lord's house and tarring and feathering them.