| Oakland | Pontiac/Michigan (USA) | 1907 – 1932 | | |
| Odetti | Mailand (I) | 1922 – 1925 | | |
| Oldsmobile | Lansing/Detroit (USA) | 1896 - | | |
| O.M. | Brescia (I) | 1918 – 1934 | | |
| Omega | (D) | | | |
| Omega-Six | Boulogne-sur-Seine (F) | 1922 – 1930 | | |
| Omikron | (D) | | | |
| OMT | Turin (I) | 1907 – 1913 | | |
| Onnasch | (D) | | | |
| Opel | Rüsselsheim (D) | 1898 - | | |
| OPES | Turin (I) | 1946 – 1948 | | |
| Orel | (F) | 1905 – 1914 | | |
| Oryx | Berlin (D) | 1907 – 1922 | | |
| OSCA | Bologna (I) | 1947 – 1967 | | |
| OTAV | Turin (I) | 1905 – 1908 | | |
| Ours | (F) | 1906 – 1909 | | |
| Overland | Terre Haute (USA) | 1903 – 1956 | | |
| Packard | Detroit (USA) | 1899 – 1958 | | |
| Padus | Turin (I) | 1906 – 1908 | | |
| Paige, Paige-Detroit | Detroit (USA) | 1908 – 1927 | | |
| Panhard & Levassor / Panhard | Paris (F) | 1889 – 1967 | | |
| Panther | Weybridge (GB) | 1972 - | | |
| Panther Diesel | Mailand (I) | 1954 – 1955 | | |
| Paramount | Swadlincote (GB) | 1950 – 1956 | | |
| Peerless | Cleveland (USA) | 1900 – 1931 | | |
| Pegaso | Barcelona (E) | 1951 – 1957 | | |
| Pennacchio | Mailand (I) | 1947 – 1948 | | |
| Pennington | (GB) (USA) | ca. 1900 | | |
| Perfetti | Mailand (I) | 1922 – 1923 | | |
| Perl | Liesing (A) | 1921 – 1927 | | |
| Peter + Moritz | (D) | | | |
| Peugeot | Beaulieu/Audincourt/Sochaux (F) | 1889 - | | |
| Peugeot-Croizat | Turin (I) | 1905 – 1907 | | |
| Phänomen (Phänomobil/Hiller) | Zittau (D) | 1907 – 1927 | | |
| Philos | | | | |
| Piccolo (später Apollo) | Apolda (D) | 1904 – 1912 | | |
| PIC-PIC (Piccard-Pictet) | Genf (CH) | 1905 – 1924 | | |
| Pieper | (B) | 1899 – 1912 | | |
| Pierce-Arrow | Buffalo (USA) | 1901 – 1938 | | |
| Pilain (Slim-Pilain) | Lyon (F) | 1894 – 1920 | | |
| Pilot | (D) | | | |
| Pininfarina ( Pinin Farina) | Turin (I) | | | |
| Pipe | (B) | 1898 – 1914 | | |
| Playboy | Buffalo (USA) | 1946 – 1951 | | |
| Pluto (Lizenz Amilcar) | Zella-Mehlis (D) | 1924 – 1927 | | |
| Plymouth | Detroit (USA) | 1928 - | | |
| Polymobil (Lizenz Oldsmobile) | (D) | 1904 – 1907 | | |
| Ponette (La) | | | | |
| Pontiac | Pontiac (USA) | 1926 - | | |
| Pope | (USA) | 1903 – 1914 | | |
| Popp | (CH) | | | |
| Porsche | Gmünd (A) Stuttgart-Zuffenhausen (D) | 1948 - | | |
| Praga | Prag (A/CSSR) | 1907 – 1947 | | |
| Premier | (USA) | 1903 – 1925 | | |
| Premier | 2 weitere Firmen in GB und D | | | |
| Presto | Chemnitz (D) | 1907 – 1927 | | |
| Priamus | Köln (D) | 1904 – 1921 | | |
| Prince | Turin (I) | 1921 – 1923 | | |
| Prinetti & Stucchi | Mailand (I) | 1898 – 1902 | | |
| Prombron (ex Russo-Baltique) | Riga (heute Lettland) | 1918 - | | |
| Prosper-Lambert | (F) | 1901 – 1906 | | |
| Protos | Berlin (D) | 1898 – 1926 | | |
| Puch | Graz (A) | 1906 - | | |
| Pup | Spencer (USA) | 1948 – 1949 | | |
| Quagliotti | Turin (I) | 1904 | | |
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