Greek compounds are usually made by combining the roots of two nouns and inserting a linking thematic vowel when it would result in a sequence of two consonants. The vowel is usually -o-, so we have e.g.
claustr-o-phob-ia, electr-o-magnet-ism, ec-o-nom-y, hor-o-scope
When the second root starts with a vowel, the linking vowel -o- is not needed, so we have
But why is "agoraphobia" with an -a-? It is made from agora and phobia, so one would expect
*agor-o-phob-ia. (but this is not the case)
And, are there more such words that use -a- not -o-?