The 80th-ranked Oudin struggled in cold and windy conditions in Birmingham, central England, failing to handle the dominant serve of Tomljanovic. The Croat will play top-seeded Kirsten Flipkens.
Tomljanovic says "my serve was a big factor. I felt really comfortable returning and that meant I could go for more on my serve."
Oudin beat Jelena Jankovic in straight sets in last year's final of the grass-court tournament, winning her only WTA title.
Eighth-seeded Maria Joao Koehler, No. 10 Hsieh Su-wei and No. 11 Ayumi Morita were also eliminated in the first round.
Results from the first day of the WTA event in Birmingham (x denotes seeding):
Marina Erakovic (NZL) bt Elina Svitolina (UKR) 6-0, 6-1
Nadiya Kichenok (UKR) bt Tmea Babos (HUN) 6-3, 6-4
Donna Vekic (CRO) bt Camila Giorgi (ITA) 7-6 (7/2), 6-3
Magdalena Rybarikova (SVK x16) bt Melanie South (GBR) 6-4, 6-2
Bojana Jovanovski (SRB x13) bt Eugnie Bouchard (CAN) 5-7, 6-3, 6-2
Ajla Tomljanovic (CRO) bt Melanie Oudin (USA) 6-3, 6-1
Kristyna Pliskova (CZE) bt Coco Vandeweghe (USA) 6-7 (5/7), 7-6 (7/5), 7-5
Alla Kudryavtseva (RUS) bt Kimiko Date Krumm (JPN) 6-4, 7-6 (7/3)
Mathilde Johansson (FRA) bt Petra Cetkovsk (CZE) 7-5, 6-2
Jamie Hampton (USA) bt Su-Wei Hsieh (TPE x10) 6-2, 6-3
Tsvetana Pironkova (BUL) bt Lauren Davis (USA) 6-2, 6-1
Madison Keys (USA) bt Lesya Tsurenko (UKR) 3-6, 6-3, 7-6 (7/3)
Vesna Manasieva (SRB) bt Maria Joao Koehler (POR) 6-7 (8/10), 6-0, 6-3
Alison Van Uytvanck (BEL) bt Ayumi Morita (JPN x11) 6-4, 6-0
Mirjana Lucic (CRO) bt Karolina Pliskova (CZE) 6-3, 6-2
Alison Riske (USA) bt Anne Keothavong (GBR) 7-5, 6-2