Is the transmitter necessary for a occasional diver?
No it's not, but it's a very convenient thing to have. If you don't want to splurge and pulling a guage up from your bcd doesn't bother you, then you may be happy without it. You can always add one later if you dive more often. I got mine when it came out for free, but IMO it's worth it. I hope that gives you a better idea of what you want.
The transmitter is not needed to use this computer. The transmitter will broadcast the remaining tank pressure to the computer. Without the transmitter, the tank pressure will not show on the computer.
I meant SPG, not spy....
The transmitter allows the computer to show the psi remaining in the tank. as long as you have an spy, there is no need for the transmitter.
No. The transmitter would relay PSI and calculated air time remaining based on the PSI. Unless you don't have a gauge to read PSI info, the transmitter is a nice to have rather than a need to have.
If you don't have a separate SPG, yes.
I dive with the OCi as my backup computer. I use it with a fast refresh rate to record my dive data and to provide me with some general information on air consumption rate. I personally would not rely on a wireless transmitter as a SPG replacement. Therefore, I do not think it is a necessary piece of equipment. I think for the occasional diver an OCS, without the wireless transmitter, would work nicely. I am very happy with the computer and have used it on several dives already. If you are a responsible diver and check your gauges often, the wireless feature is a luxury not a necessity.