You mean like using a sequence to generate numbers also with a prefix:

class crm_lead(models.Model):
    _inherit = 'crm.lead'
    name = fields.Char(default='/')
    
    @api.model
    def create(self, vals):
        if vals.get('name', False) == '/':
            vals['name'] = self.env['ir.sequence'].next_by_code('crm.lead.seq')
        return super(crm_lead, self).create(vals)

Depending on the Odoo version(ex: v8) you need to include the sequence type like:

<record id="sequence_crm_lead_type" model="ir.sequence.type">
    <field name="name">CRM Lead Sequence Type</field>
    <field name="code">crm.lead.seq</field>
</record>

And add the sequence record

<record id="sequence_crm_lead" model="ir.sequence">
    <field name="name">CRM Lead Sequence</field>
    <field name="code">crm.lead.seq</field>
    <field name="prefix">OP</field>
    <field eval="1" name="number_next"/>
    <field eval="1" name="number_increment"/>
</record>

 Hope this helps

Add the ID field to the views - it is already a unique number assigned automatically by Odoo to each document (the database id).