Oath against Fiends and Oath of the crusarder.

[Ignacio Arregui]

Can a paladin have those both oaths?

Both of them change the Mercy quality but at diferent levels.

[Mysterious Stranger]

Oath of the Crusade
This oath is identical to the oath against fiends except as noted here.

Technically you cannot take both at once, but you don’t need to. The Oath of the Crusade gives you everything the Oath against Fiends gives you. The Oath of the Crusade is a sub archetype that replaces additional class features.

[Azothath]

Paladin Oaths An oathbound paladin can take multiple oaths, but none of the other oaths can replace or alter the same class feature from the paladin class as another alternate class feature. For example, a paladin cannot take the Oath against Corruption and the Oath against the Wyrm because they both replace the aura of courage class feature.
no - as they alter/replace the same class feature.

[Mysterious Stranger]

The Worldwound threatens all of Golarion with the uttermost depths of depravity, and so paladins across the world stand firm against it. This oath is identical to the oath against fiends (Pathfinder RPG Ultimate Magic 61) except as noted here.

@Azothat you are technically correct, but in this case it does not matter because the Oath of the Crusade gives all the abilities that the Oath against Fiends gives you, plus to more abilities. The features the Oath of the Crusade gives do not modify or replace any abilities from the Oath against Fiends.

The Oath against Fiends replaces the Aura of Resolve and the Mercy gained at 9th level. The oath of the Crusade alters divine bond and adds an additional mercy that the paladin can choose at 12th level.

[Azothath]

the variant Oaths are accumulative. Oath of the Mendevian Crusade (OMndCru) contains the Oath against Fiends (powers) just as the Oath against the Whispering Way(OWspWay) contains the Oath against Undeath(OUnD) (powers). The OWspWay contains a change to the OUnD which clarifies the cumulative nature of the variants by gaining Aura against Necromancy in place of Aura of Life by replacing Aura of Resolve. Adding the bolded name of the powers from the primary oath would have made it obvious in the variants.

So no, a PC cannot take both as taking the second oath (again) would alter what the first altered (for primary or variant oaths in any order). The variant Oath includes the other Oath BUT the Paladin has to abide by the code of conduct in the variant. A generic statement in the variant like "oppose undead" could be detailed in the primary oath but that's a GM call. So in a practical way the paladin takes both when he takes the variant (if that's what you mean) though there can be changes.
I think it might be better to say the variant replaces/supplants the primary oath if both are taken.

[Ignacio Arregui]

Thanks a lot