Hewlett-Packard Co yesterday cut list prices on its HP OmniBook 5500 notebooks by as much as 21%, and also cut prices on its OmniBook 800 notebook and the CD-ROM accessories for the HP OmniBook 5500. The company claims the cut places it in strong competitive standing with other full-featured 133MHz Pentium notebooks on the market. Estimated street prices are expected to start at $1,530 for the HP OmniBook 5500CS 5/100 notebook model 810, a 100MHz Pentium system with an 810Mb hard disk drive and an 11.3 viewable-area passive-matrix display.