Well, building a Blaster in this sort of group composition is probably the worst thing this party needs, since they have a distinct lack of out-of-combat healing, as well as condition removal and buffs. In even the most basic of campaigns, lacking buffs, condition removers, and out-of-combat healing has resulted in PC deaths and/or TPKs.
Consider the adventure path name "Mummy's Mask." Mummies usually have a bad disease effect called Mummy Rot when they hit an enemy. Mummy Rot is a Cursed Disease that, without proper condition removals (of which there are no mundane means to fix), will eventually outright kill a character. This might not be an issue if your Fighter was a Paladin, and your Magus was a Barbarian, but realistically speaking your Fighter and Magus will probably get hit, and may easily suffer from those conditions and die without Remove Disease and Remove Curse instead of, say, Fireball and Flamestrike at your disposal.
That being said, multiclassing in this case is a waste of time for what little you get out of what potential you can get from your Oracle features, and Mystic Theurge isn't going to be doing you any favors here since Mystic Theurge is generally bad (and for good reason).
Casters almost never benefit from multiclassing. Concentrate on your character's strong points and use your spell selection to its fullest. As Darksol says, you'll need lots of condition removal quite frequently in an undead-themed campaign like this one, and shouldn't need to blast much anyway.
I know it's frustrating to have a spellcaster who doesn't blast his enemies, but that's simply not your strong suit.
Also: learn how to ration your spells more. Have a backup plan for doing something useful, even if it's only plinking away with a light crossbow. Save your spells for when they are most needed.
I agree with Darksol and Wheldrake. You are better suited to remaining single classed to be most benefit to the party. You have a magus, your blasting won't really be needed.
If you want to be a Mystic Theurge in this party, you have a steep hill to climb, unless you can remake your character. If you want to take a SINGLE level of sorcerer to get that Crossblooded (and the +2 damage per damage die ability), it might be worth it but you will need to invest in scrolls and consumables to make up the loss of spell levels.
Not recommended. There is almost nothing that is worth a full caster losing a level of spell progression.
I am not sure what level you are (I assume 1st level) but it might not be too late to pick up an archetype for your Oracle or a variant Multiclass (if allowed). The Spirit Guide Oracle adds quite a bit of flexibility to the Oracle chassis, and picking up Sorcerer (or Wizard while taking EH) could help with spell flexibility, school/bloodline powers to help fill in the gaps.
Also 1st level character often run out of spells and need their back up weapon or cantrip , it happens all the time, as you reach higher levels you will have more and more spells and can fall back on low level spells for those encounters when you don't want to expend a high level spell.
Spirit guide Oracle can be complicated and there are some rules which have to be interpreted by each GM specifically the Arcane enlightment hex is very controversial with some ruling it gives you nothing, others saying it lets you pick sorceror spells differently each time you take it and others such as me letting it give you access to a fixed list of sorceror spells.
So before taking Spirit guide talk to your GM so you don't have different interpretations when it matters
Spirit guide Oracle can be complicated and there are some rules which have to be interpreted by each GM specifically the Arcane enlightment hex is very controversial with some ruling it gives you nothing, others saying it lets you pick sorceror spells differently each time you take it and others such as me letting it give you access to a fixed list of sorceror spells.
Best solution to that is just not bother with those 'complicated issues' and choose different abilities that don't complicate matters. Just stay away from the Lore Spirit.
Spirit guide Oracle can be complicated and there are some rules which have to be interpreted by each GM specifically the Arcane enlightment hex is very controversial with some ruling it gives you nothing, others saying it lets you pick sorceror spells differently each time you take it and others such as me letting it give you access to a fixed list of sorceror spells.Best solution to that is just not bother with those 'complicated issues' and choose different abilities that don't complicate matters. Just stay away from the Lore Spirit.
especially when there's a FAQ clarifying it doesn't work for oracles
I like multi-classing because it is fun.
What everyone says is true about spellcasters... splitting levels slows down spell progression. But I would hasten to add that multi-classing can toughen up a character or contribute in other ways that make them stronger than going for more spells.
With three person party that has no cleric or rogue going into crypts, for an adventure that is ideal for four member parties... your character will be vulnerable. You can toughen up and contribute in other ways.
With your strength so weak and -4 on melee because of the blackened curse, there is no point in talking about melee options. But you do have a 13 dex... a ranged combat build would help you contribute even when spells run out, or save the blasting for when it really matters. That could be an archer fighter, or a far strike monk who flurries with thrown daggers, or a ranger. Taking a sniping rogue or ninja could be interesting as well... the whole rogue/magic thing is always fun to play, and it could help you detect and evade traps.
Good luck!
Sounds like you have a fun character!
Ask your GM if you can take the Leadership Feat at Lvl. 7
Then get a cleric as Healer/buffer and go to town:-)
The feat is ideal for small groups like yours and shouldn't unbalance the campaign -
good Gaming!