Paroxetine belongs to a class of medication known as selective serotonin-reuptake inhibitors (SSRIs) and it is licenced in the UK to treat a range of depression and anxiety disorders, including OCD, panic, etc. Which means it targets a range of conditions and not just one. Mirtazapine is a presynaptic alpha2-adrenoreceptor antagonist which increases central noradrenergic and serotonergic neurotransmission, and is licences in the UK to treat major depression. The combination of both medications is seen to help select levels of serotonin which is a neurotransmitter (a messenger chemical that carries signals between nerve cells in the brain), and it is believed to have good influence on mood, emotion and sleep.
Serotonin-norepinephrine reuptake inhibitors on the other hand completely block both serotonin and norepinephrine neurotransmitter. These class of medications are more appropriate for acute symptoms and not long term issues, because they can cause long term side effects including triggering manic or hypomanic episodes.
Medication needs to be taken with caution and you need to also take something to help line your stomach, like Omeprazole, because medication can cause gastric problems in the stomach and related organs.
Also, medication is best used with psychological treatments, to help people identify triggers for the symptoms and better coping mechanisms during relapses.