OVA by its name is just the pure protein ovalbumin, NP is a small chemical that conjugated to OVA so when mice are immunized, they will generate both NP-specific and OVA-specific B cells but the NP-specific are the dominant population please see this paper: https://www.cell.com/cell-reports/pdfExtended/S2211-1247(17)31810-7 (Targeted Elimination of Immunodominant B Cells Drives the Germinal Center Reaction toward Subdominant Epitopes). OVA alone immunization will generate strong OVA-specific T cell responses but for B cells its harder to detect OVA-binding on B cells than NP-binding, the staining is dimmer. NP-binding is a widely-accepted model system to study B cell in immunization especially heavily used for germinal center and plasma cell studies.