Bruce Neagle is absolutely right, GC/NPD are the top of the top, but maybe you don't need such analytical power, which concentration range are you going to investigate?
Sorry Bruce and Andrea, y'all are both really, really wrong. HPLC-MS/MS is by far the most sensitive technique for o-P pesticides. However, HPLC-UV simply won't work. You'll get way too many interferences extracted from the animal tissue unless you go through extensive and exhaustive cleanup procedures. Move to HPLC-MS/MS and use conventional extraction and cleanup procedures. You'll need a relatively polar column (we use the Restek Ultra Aqueous) to get the front end pesticides separated, and it will help with the remainder of the o-P pests.
I agree with my colleagues that you will probably need a more sensitive detector than UV (e.g. QqQ). Regarding sample preparation, the Quecher method could be a good choice for animal tissue. In the following links you can find some examples:
I would assume that you want to analyze OP with the instrument you have. I agree, the ideal instrument is either GC/MS or LC/MS, but if you don't have it then it is no use. You can ride a bike from India to China, but it will take a while. Depending on the level and I hope that it is high enough. You have to do a lot of sample cleanup to get rid of interference that have similar UV absorbance characteristics as your OP. I found a good SPE MIP that will trap only OP and release other stuff. You may use QuEChERS extraction and cleanup first for simplicity. However, it is too dirty for UV, so you need the SPE MIP to make the extract cleaner.