How do you say Ogham?
I wish you hadn't asked that.
It can be ogg'm; oh-'m; awm; ohm; or it can have a sound like an h only spittier and garglier in the middle: ohh'm.
What is Ogham script?
That's a bit easier. It's an alphabet consisting of straight lines cutting across or into a vertical line, sometimes at right angles, and sometimes diagonally. Quite often the vertical line is a corner of an upright stone:
The script was used by Celtic people from about the fourth century onwards (perhaps earlier) and usually the inscriptions give a name, with the implication that the named person owned the land, or was perhaps buried under it. Most of the inscriptions are found in Ireland, but Wales and Scotland and England have examples, too.
Sometimes Ogham script is found on portable objects, too. Here's a modern alphabet: