just bought a new raceface x-type bb whent to fit it onto my on one ss and noticed ive got an FSA octalink BB do i need to buy a new chainset

The FSA chainset will be ISIS, not Octalink (proprietary Shimano).

You need a new ISIS BB or an external-bearing crankset…

No guesses which will be cheaper at this time.

Both are not long-lived improvements on Square Taper, but that would require you to buy both crankset and a BB.

FSA do (or did) make octalink compatible chainsets.

“do i need to buy a new chainset”

what condition is it in , how worn are the teeth on the chainring?
Why did you buy a new bottom bracket?

bottom bracket was knackered so ive just taken the cheaper option and bought a new bb the x-type fits my Giant thanks for the advice guys not a mistake i will make again