| 61. | .org | Calendar file (Lotus Organizer) ◊ Origin plot (Microcal Origin) |
| 62. | .orig | Backup file |
| 63. | .oss | Search result (Microsoft Office) |
| 64. | .ost | Offline file (Microsoft Exchange/Outlook) |
| 65. | .otl | Outline font description (Z-Soft Type Foundry) ◊ Ventura Publisher's Type Foundry Outline Editor format |
| 66. | .otx | Text file (Olivetti Olitext Plus) |
| 67. | .out | Output |
| 68. | .ov | Database file (Revelation-DOS) |
| 69. | .ov1 | Overlay file (part of program to be loaded when needed) |
| 70. | .ov2 | Overlay file (part of program to be loaded when needed) |
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