Wide Field Slitless Spectroscopy (WFSS) provides a powerful tool for detecting strong line emission in star-forming galaxies (SFGs) without the need for target preselection. As part of the GLASS-JWST Early Release Science (ERS) program, we leverage the near-infrared wavelength capabilities of NIRISS (1-2.2{mu}m) to observe rest-optical emission lines out to z~3.4, to a depth and with a spatial resolution higher than ever before (H{alpha} to z<2.4; [OIII]+H{beta} to z<3.4). In this Letter we constrain the rest-frame [OIII]{lambda}5007 equivalent width (EW) distribution for a sample of 76 1