The spelling of "Olliver" may seem unusual, but it can be explained using IPA phonetic transcription. The correct pronunciation is /ˈɒlɪvər/. The letter "o" represents the sound /ɒ/ as in "pot." The double consonant "ll" indicates a long /l/ sound, as in "tall." The letter "i" represents the short /ɪ/ sound, as in "sit." Finally, the letter "e" is silent in this word. So, "Olliver" is a perfectly legitimate spelling for the name pronounced as /ˈɒlɪvər/.