| OC | open circuit | Opposite of short circuit. |
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| OCR | optical character recognition | Scan in a printed document and voilà, you have a zillion mispellings to correct. Ask Walter Rotman.... |
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| OCXO | oven controlled crystal oscillator | -- |
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| OD | outside diameter | The outside dimension of a cylinder. |
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| OE | Oersteds | Unit of magnetic intensity, named after Hans Christian Oersted. |
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| OF | Old fart | That old guy out in the lab that knows everything but has hairs growing out of his ears. |
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| OFDM | orthagonal frequency division multiplexing | A method of sending multiple closely spaced carrier signals from a single transmitter. |
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| OFHC | oxygen-free high-conductivity (copper) | The ideal material for low loss waveguide. |
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| OMN | output matching network | -- |
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| OMT | ortho-mode transducer | -- |
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| OQAM | offset quadrature amplitude modulation | -- |
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| OTA | over the air | -- |
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| OTOI | output third order intercept | -- |
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