I'd like to start doing some low temperature OA whuzzit fixits. Just enough to get some bonding metal to hold the thing together, max working pressure
400 psi. Like soldering, but just a little stronger. Is there a guide for me to flux coated wire electrodes that might help me with my selection? I figure that a supply of four different fluxes, and four different diameters would cover most situations, and most probably a lot less than the 16 permutations you get from those variables.
If you had to pick four rods and flux combos to cover 75% of common situations, what would they be?
You can "solder" using tin, lead, or silver alloys.
Low temp soldering on copper and steel is done with tin or tin/lead alloys. There are acid paste fluxes, but the liquid zinc chloride flux works the best. The solder is sold in 1 lb. spools of different diameters, depending on the scale of the work you are doing. You can also get "acid-core" solder that is a hollow tube with the flux inside. Hobbyists use it, professionals don't. Be careful of the fumes, they can be nasty. These alloys of solder are intended for soldering copper water pipe using the pure Tin alloys or steel sheet metal using the Tin/Lead alloys.
Next up in strength are the low content silver solders known as "silver-bearing" or "soft" solders. They are still mostly tin or tin/lead, with enough silver to increase the strength while still allowing them to flow at temperatures below 700 degF. These can be used for higher strength repairs than the tin or tin/lead solders.
Brass, bronze, copper, and steel can be "brazed" using LF (low fuming) bronze, and a flux. The rods can be purchased pre-coated with flux, but these have a short shelf life as the flux will absorb ambient moisture and break down. The better way to go is to purchase 1 lb. cans of flux. There is a standard flux used for most applications and a high temperature flux used for brazing cast iron. Warm the end of the rod with the torch, and dip it into the powdered flux to adhere it. Reapply flux at regular intervals as you braze.
High temp soldering is done using high content silver solders (aka "hard solders") and is often referred to as "silver brazing". This is used for gun parts, knife hilts, and most silver jewelry. The silver solder can be ordered in many different temperature ranges so you can solder multiple times on the same part by moving down in temperature each time. The 3 main ranges are Hard, Medium, and Easy. Hard silver solders use a higher temperature flux than the silver bearing solders. I like the Sta-silv product line of paste fluxes for work on copper alloys, steel and stainless steel. For actual jewelry work Batters flux works very well.
Be careful with the hard silver solders. Silver is a much better heat conductor than copper, so don't try holding the end of the solder with your bare hand, use pliers or forceps.
Applying solder with a long length of wire is good for low temp work, but for high temp solder I prefer "chip" soldering where short pieces of solder are snipped off and applied to the joint using a small brush dipped in flux. The entire area is then carefully heated to first dry out the flux to a white crust, and second to melt that crust and the solder until it flows. This is a tricky task and tiny solder chips will often pop off the joint as you boil the water out of the paste flux.
Solder can only flow where there is flux removing the oxides.
There are also aluminum solders. The simplest are the "miracle" rods which are mostly zinc. they can work for simple low strength repairs on many metals, but they have no real strength and the joint is quite brittle. Handy for patching a hole in a aluminum canoe or water tank.
Higher strength aluminum solders such as Alstate #30 comes with a matched flux. It is used mostly for aluminum refrigeration, and air lines.
You need to get the work quite hot to braze with brass - this can sometimes just about be done with a propane/air torch, or better a mapp/air or mapgas/air torch, but it can be a real problem if you don't have a big enough torch, and/or a large selection of torches.
Silver soldering, also known as hard soldering or silver brazing  is done at a slightly lower temperature, about 670 C instead of 950 C, and is a good bit easier as far as torch selection goes .
Soldering with propane/air is eminently practible for the lower temperature silver solders, and possible even for the higher temperature ones, though MAPP/air or mapgas/air is better there.
The OP was talking about OA, which doesn't really have that problem - and even oxy-propane is okay, for both brazing and silver soldering, if the work can't take the heat of brazing.
Oxy-propane can be very useful when getting hold of acetylene is a pain and you don't need it to weld with.
Silver solder is a lot more expensive than brazing brass (which isn't exactly cheap either) as it contains a goodly bit of silver, but it is plenty strong enough for almost anything at 180 MPa shear 400 MPa yield
500 MPa UTS in as-soldered state (that's like a good mild steel, and with three times the shear and yield strengths of as-brazed brass), and for small jobs it is still affordable.
It's a bit too expensive for large jobs though - not so long ago I repaired the mounting of a washing machine drum, used about $25 worth as I had to go all around the mounting. Wouldn't want to use silver solder for anything much larger, unless it was already expensive stuff, eg joining LOX pipes.
Silver soldering stainless can be a little tricky, but really you just need the right materials. In the UK attention is focussed more on the flux, while I believe in the US more attention is placed on the solder, but really just get a combination which is meant for stainless and experiment a little, it isn't hard if you use the right stuff.
 the names vary. I mean joining with a filler containing 56% to 24% silver and a melting point of 620 to 780 C.
When they banned lead in water pipe joints a new form of solder was introduced containing 3-5% silver, a smidgin of copper and the rest tin, with a mp of about 220 C. Similar solders are used in electronics after RoHS. while both are sometimes called silver solder, they are soft solders with only 5% of the strength of the hard silver solders (25 MPa vs 500 MPa) and they are not at all what I am talking about.
There are about four different formulae for hard silver solder in common use, with melting points differing by about 50 degrees, so you can step braze: solder using a high temp solder then solder again with a solder with a slightly lower MP and not melt the first joint - step brazing can also be done using diffusion brazing, but you'll need an accurately electronically temperature controlled furnace to do that one properly.
There are also cadmium-bearing silver solders, which are slighly cheaper, have slighly lower mp's, flow better and which wet some things better (but others worse); but they have recently been banned in the EU, and I never much liked using them anyway.
 At these temperatures most of the heat is lost through radiation, and the radiant heat loss goes up as the fourth power of absolute temperature, so to go from 670 C to 950 C you need to overcome nearly three times (2.83744 x) the heat loss.
Also, the difference between the flame temperature and the work is less, so the transfer of heat from flame to work is less (about half as effective), so overall to reach 950 C in the work the flame has to be at least six times the power needed to reach 670 C.
That's some of the theory - in practice you'll probably need even more torch power, say ten times, as you need to reach temperature in a reasonably short time.
For OA or oxy-propane the difference in power needed is less, as the flame is hotter and therefore the difference between flame temperature and work temperature is higher, so heat transfer between flame and work is more efficient. Recommended for brass brazing, or the higher temperature silver solders.