| O`ROURKE | James | well digger | Springdale | Butler | |
| O'BRIEN | James | machinist | Main | Karns City | |
| O'BRIEN | M | of Hersh & O'Brien | boards European hotel | Millerstown | |
| O'BRIEN | Simon | helper | boards Race | Millerstown | |
| O'BRIEN HOUSE | T J O'Brien | proprietor | Slippery Rock | Millerstown | |
| O'CONNER | John | Main | Greece City | ||
| O'CONNOR | T | bakery | Main | Millerstown | |
| ODELL | C | widow | Franklin | Butler | |
| ODELL | J A | driller | boards Adams house | Fairview | |
| ODELL | W H | contractor | boards Adams house | Fairview | |
| O'DONNELL | M J | oil operator | boards Central house | Petrolia | |
| OESTERLING | Fred | of J OESTERLING & BRO | Jefferson | Butler | |
| OESTERLING | James | of J OESTERLING & BRO | boards Vogeley house | Butler | |
| OESTERLING J & BRO | livery | rear Vogeley house | Butler | ||
| OETTINGER | Frederick | saddle and harness maker | Main | Millerstown | |
| OHL | Adam | Saxonburg | |||
| OLHMAN & KINGSBACHER | clothing | Main | Millerstown | ||
| OLIVER | Charles | clerk | Jamison | Petrolia | |
| OLIVER | George | Portersville | |||
| OLIVER | J E | confectionery | Main | Petrolia | |
| OLSON | John | tailor | North | Butler | |
| O'NEIL | H F | secretary Keystone Pipe Co | boards Main | Millerstown | |
| OPERA HOUSE HOTEL | CLARK, Isaac | proprietor | Main | Karns City | |
| ORAND | Miss Elizabeth | tailoress | boards Church | Petrolia | |
| O'ROURKE | Edward | tool dresser | boards Sherman house | Millerstown | |
| O'ROURKE | S F | contractor | boards O'Brien house | Millerstown | |
| ORPHANS' HOME | Millerstown road northwest Butler | Butler | |||
| ORR | Ann Eliza | widow | North | Butler | |
| ORR | John P | photographer | east Jefferson, res. west North | Butler | |
| ORTON | O | teamster | boards Apollo house | Karns City | |
| OSBORN | B H | oil dealer | Franklin | Butler | |
| OSTERLING | Peter | Main | Zelienople | ||
| OTT | Joseph | helper | Mill | Millerstown | |
| OTTERSTATLER | J H | book-keeper H & L Taylor & Co | boards Wolcott's | Petrolia | |
| OTTO | carpenter | boards Monitor house | Petrolia | ||
| OTTO | Christian | west Cunningham | Butler | ||
| OTTO | Peter | merchant | Mercer | Harmony | |
| OTTO | William | laborer | west Cunningham | Butler | |
| OUGHTON | James | Main | Greece City | ||
| OVERY | Joseph | president Keystone Pipe Line Co | residence McLaughlin farm | Millerstown | |
| OXLEY | W H | clerk | Main | Petrolia |