| Competition: | NSL Second Division |
| Season: | 1991 |
| Winners: | Cape Town Spurs |
| Prevseason: | 1990 |
| Nextseason: | 1992 |
The 1991 NSL Second Division (known as the OK League for sponsorship reasons) was the seventh edition of the second tier competition organised by the National Soccer League. It was divided into two divisions known as streams.
| Club | City | Home venue | |
|---|---|---|---|
| Benoni Wanderers | Benoni | Northmead | |
| Bluebells United | Lenasia | Lenasia | |
| Boksburg | Boksburg | Prince George Park | |
| De Beers | Johannesburg | George Goch | |
| Fordsburg City | Johannesburg | Union | |
| Health Centre Utd | Bushbuckridge | Pauliana | |
| Jasair Birds | Pretoria | Laudium | |
| Leicester City | Klerksdorp | Alabana | |
| Kroonstad Rabbits | Kroonstad | Seisoville | |
| Manchester City | Benoni | Wattville/Actonville | |
| Neac All Stars | Dobsonville | Dobsonville | |
| Pretoria Callies | Pretoria | Super | |
| Ratanang | Phuthaditjhaba | Phuthaditjhaba | |
| Ravens | Bloemfontein | Seisa Ramabodu | |
| Real Rovers | Potgietersrus | Mahwelereng | |
| Rockridge | Johannesburg | Orlando | |
| Sparkling Monarchs | Wonderkop | Wonderkop | |
| Swaraj United | Johannesburg | Lenasia | |
| Vaal Professionals | Sharpville | George Thabe | |
| Zola United Stars | Johannesburg | Orlando |
| Club | City | Home venue | |
|---|---|---|---|
| Battswood | Cape Town | Athlone | |
| Berea | Durban | Chartsworth | |
| Blackburn Rovers | Butterworth | Umsobomvu | |
| Bush Bucks | Durban | Glebe | |
| Cape Town Spurs | Cape Town | Parow | |
| Coronation | Durban | Curries Fountain | |
| Crystal Brains | Durban | Umlazi | |
| D'Alberton Callies | Durban | Curries Fountain | |
| Dynamos | Newcastle | KR Rumelin Stadium | |
| E.L. Bluebells | East London | ||
| Hotspurs | Port Elizabeth | Adcock | |
| Manchester United | Escourt | Forderville | |
| Maritzburg United | Pietermaritzburg | Northdale | |
| P.E. Callies | Port Elizabeth | Adcock | |
| Real Aces | Vryheid | Hlobane | |
| Royal Tigers | Durban | S J Smith | |
| Stanger United | Stanger | Stanger | |
| Tembu Royals F.C. | Umtata | Independence | |
| Umtata Liverpool | Umtata | ||
| Zulu Royals | Durban | Kwa Mashu |
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Cape Town Spurs won the Championship.
A new cup for the NSL Second Divisions teams was introduced in 1991 sponsored by the New Republic Bank. Sixteen clubs in total entered the competition, 8 from each stream.
Cape Town Spurs won the Knockout Cup.