The Oxen blocks are not the standard blocks on an 18, but some people prefer them as they are usually easier to uncleat. One of the problems with them is that there is a part inside that eventually fails and then does not allow the block to release. If you intend to use the Oxen blocks, it a good idea to send them back to the manufacturer and have them rebuilt (I think you can do this throug Murrays).
Originally, 18's came with Seway jib blocks which are horrible when it comes to uncleating. The later boats came with Harken's which are much nicer. If you go to the Hobie web page, you should be able to access the parts guide in the online catalogue which calls out the Hobie part number (60330041) for the Harken block. Otherwise, contact your Hobie dealer.
Basically what you're looking for is a single rachet block with a becket on top and a cam cleat. It can be seen in the murrays catalogue under the jib sheeting section. If you still have the slider and stand-up spring from the original block, you can re-use that. Otherwise you will need a complete assembly. Murrays lists the block alone for about $140.
sm
