We investigate the atomistic structure of ferroelastic-ferroelectric 90 degrees domain walls in PbTiO3 with first-principles calculations and high-resolution scanning transmission electron microscopy. We find sharp discontinuities in the variation of lattice parameters across the domain walls. Unexpectedly, the two neighboring domains become asymmetric across the boundary, giving rise to primitive unit cells with large tetragonality ratios (c/a) of the order of 1.11 close to the boundary. The variation of the domain wall structure with respect to strain is demonstrated. We show that oxygen vacancies are attracted to the plane adjacent to the 90 degrees domain wall. The mechanisms of domain wall pinning by oxygen vacancies is explained based on the energy landscape of the vacancies in the presence of the domain interface.