Strictly phonologically speaking, most English and other Germanic words we think of as beginning with a vowel or silent h actually begin with a glottal stop when spoken individually. Uh-oh, to take an example from a previous writeup, contains two glottal stops, one before each vowel. Feel your voice box when you speak it if you don't believe me. Unless you're a native speaker of a language that doesn't regularly use glottal stops, the two vowel sounds are almost definitely separated by a voiceless consonant articulated by closing your glottis.
Glottal stops also appear more frequently in many English dialects (try a Scottish "accent" on for size) where more "standard" dialects use alveolar stops like d, t, or flaps.