Do these instruments have similar origins and purposes? They look almost identical, but somewhere I read that the English horn is larger and is, IIRC, “a pitch lower than the oboe.” That makes me think the English horn is identical in tone to the oboe, just lower in tone.
Is a pitch an octave?
What are the key differences between these two woodwinds?
A “pitch” lower doesn’t mean anything. A pitch is the musical term for a frequency; it is an absolute measure. For example, A is a pitch, and its frequency is 440 Hz. Differences can be expressed as half and whole steps, or Hz. B is a pitch. A is one whole step lower than B. However, the English horn is lower pitched (not a pitch lower).
I do not know about the history of these instruments but they are both double reeds. In a double reed the sound is made by two reeds vibrating against each other. (A clarinet, in contrast, has a single reed.) There is also the bassoon, an even lower pitched double reed.
I also don’t know what keys these instruments are in but that will tell you exactly the difference in pitch.
If you do a Google search on “history oboe english horn”, you’ll find sites aplenty pointing out that the English Horn is an alto oboe, so it’s the same technlogy, just a bit bigger so that it plays a 4th or 5th below the oboe. One site points out that they were based on the shawm, but modified for indoor use. Good call. Those suckers would take the powder right of your wig.
The English horn is in F, so a written C sounds as the F a fifth lower.
In the UK, “oboe” means “tramp.”
In the US, “hobo” means “a large serving of ice cream.”
Biffy the Elephant Shrew is correct. One of the more crucial differences between the two instruments is that the English Horn will burn a bit longer than the oboe, being a somewhat larger instrument.
…though neither will burn quite so long as a bassoon.
To elaborate - the cor anglais (the common English name for the English Horn, just to be awkward) has a lower compass than the oboe.
Instruments are not ‘in keys’. They are chromatic. Some (but not any double reeds) are notated in transposed keys, but this doesn’t tell you anything about their compass.
Beg pardon? Transposing instruments most definitely are ‘in keys’. The English Horn, as mentioned above, is in the key of F. Meaning a written C will actually sound as the F a fifth lower. Clarinets come in the keys of Bb, A, and Eb, string instruments are in C, etc.
Perhaps you are confusing the meaning of the word ‘key’ as it applies to transposing, and as it might apply to an instrument that can only play in one key, such as a toy piano or a viola. 
A trumpet is “in Bb”, but not “in the key of Bb”. The music has a key, not the instruments.
And yeah, I forgot the cor was transposing :smack:
Very true, my bad. I was not familiar with the term “compass”, and I got sloppy.
But wait, you just said:
As for putting an ‘in the key of’ in front (instead of just ‘in’), it’s perfectly acceptable.
Thanks for the update, professor.
*An American violinist walks into a music store in England, and tells the clerk he needs to buy an ‘E’ string.
The clerk goes back and comes out with a big box of violin strings.
“Find it yourself”, the clerk says. “I can’t tell an ‘E’ string from a ‘She’ string”*
Is the oboe as hard to play as I’ve heard?
Acceptable? It’s certainly incorrect. “In Bb” is not a designation of a key, it’s a designation of transposition. Which, in any case, has nothing to do with the structure of the instrument, and exists purely in its relationship to notated music and to other instruments.
Just doing my bit to dispell ignorance. And no respectable professor would give a straight answer, anyway 
Bet he could tell it from a G-string.
As in “a whole bowl”?
It makes more sense if it is “in the South”.
I don’t understand what a “4th or 5th below the oboe” means. How would that compare to, say, an octave?
Indeed. Our babysitter, when I was a wee Nott, told us a joke about, “But, mama, how can we have mo’ lasses, when we ain’t had no 'lasses yet?” She was from Mississippi. This is a surprisingly diverse town, for central Indiana. I know folks born in Kyoto, Talledega, Chattanooga, New Orleans, Kashmir, Montreal, Amarillo, Cairo (Egypt,) East Los Angeles, and Providence, Rhode Island.