You can pay into more than one pension scheme at the same time.
If you are paying into a personal pension then your employer is not paying any money into it. However, if you contribute to OSPS the employer contributes a significant amount as well.
When you leave you should be able to transfer your OSPS benefits to your personal pension. A year’s membership of Investment Builder in tier one at £20,000 per annum salary could generate a fund value of around £2,000 (actual figure will depend on investment returns and charges). This would cost you £640 net allowing for tax relief at 20%.
Use the Investment Builder calculator (xlsx) to see how it works for you.